v2.0.0-rc2: Merge pull request #214 from mbdavid/szurgot

New features in RC2

• Portable version: NuGet package: Lite.Core that support PCL/UWP
• New disk access (for safe concurrency) implementation
• Transactions are back
• No more ThreadSafe (only ProcessSafe). Always create LiteDatabase instances.
• Close datafile after each command execution
• BsonMapper.Global are back
• RegisterAutoId<T> are back

Changes in RC2

• To implement new disk read/write access, new datafile version was required.
• Removed OnModelCreating. Use BsonMapper.Global instance
• Removed UserVersion update checking. Now UserVersion is only an int stored in header page (update version must be developed in your solution)