Added support for filling properties not defined in model

[esentio]

This PR fixes #33.

This allows to fill entity properties not defined in model in SELECT queries using Include methods.

For example:

using (var db = CreateContext())
{
    var list = db.From<Article>()
                 .SelectAll()
                 .Include(x => x.PriceWithDiscount, x => x.Price * 0.9m)
                 .ToList();
}

There will be a new column in SQL resultset with Price * 0.9 value and this value will be assigned to Article.PriceWithDiscount property.

VB.NET even allows to use nicer assignment syntax:

Using db = CreateDbContext()
  Dim list = db.From(Of Article).
                SelectAll().
                Include(Sub(x) x.PriceWithDiscount = x.Price * 0.9D).
                ToList()
End Using

Every expression used in Include method will be added as an additional column (or multiple columns) to the SELECT statement. If you don't need columns automatically added by SelectAll method, you need to exclude them:

// Exclude ArticleCategory columns to make GROUP BY work:
using (var db = CreateContext())
{
    var list = db.From<Category>()
                 .LeftJoin<ArticleCategory>(j => j.T1.Id == j.T2.CategoryId)
                 .GroupBy(j => j.T1)
                 .SelectAll()
                 .ExcludeT2()
                 .Include(j => j.T1.ArticleCount, j => Yamo.Sql.Aggregate.Count(j.T2.ArticleId))
                 .ToList();
}

// Assuming: modelBuilder.Entity<Article>().HasOne(x => x.Label);
// Exclude Label columns to not create Label object and to not assign it to Article.Label property.
// Only Article columns and Label.Description column will be in the resultset.
using (var db = CreateContext())
{
    var list = db.From<Article>()
                 .LeftJoin<Label>(j => j.T1.Id == j.T2.Id)
                 .SelectAll()
                 .ExcludeT2()
                 .Include(j => j.T1.LabelDescription, j => j.T2.Description)
                 .ToList();
}

// If ExcludeT2() were not called, Label columns would be included twice (and Article.Label property would be filled).
using (var db = CreateContext())
{
    var list = db.From<Article>()
                 .LeftJoin<Label>(j => j.T1.Id == j.T2.Id)
                 .SelectAll()
                 .ExcludeT2()
                 .Include(j => j.T1.Tag, j => j.T2)
                 .ToList();
}

You can include simple scalar values like, whole entity (model object), ValueTuple (although only VB.NET supports this) and anonymous objects (probably useful only in limited number of use cases due to casting issues).

NOTE: if whole entity is included, it's always a "detached copy" unrelated to what would normally work using SelectAll method. This means:

• any Exclude method on that entity is ignored for Include entity instance (it always contains all columns defined in model configuration)
• no relationships are set for Include entity instance
• Include entity instance is not used in any relationship

If you don't need a "detached copy", it's probably just better to use As method to define an ad hoc relationship.

NOTE: Include is only available "in automatic mode", i.e. when SelectAll() is used. It's not supported in custom selects, i.e. when Select(<expression>) is used!