Support validation annotations on properties

[ljacqu]

Proposition:
Allow other annotations to define certain constraints, e.g. only positive numbers. Maybe we can have some implementations already for some of http://docs.oracle.com/javaee/6/api/javax/validation/constraints/package-summary.html

[ljacqu]

Closing as wontfix as it's easier to have validation in a PropertyType or Property implementation that discards the value of the YML if it's invalid so that ConfigMe uses the default, e.g. if we only allowed positive numbers a type impl could easily call reader#getInt and then still return null from BaseProperty#getFromReader if the value is <= 0, for example.
I think this keeps everything clearer and doesn't add an additional layer of complexity that one must keep in mind.
I'm not sure if I rather had bean properties in mind while writing this but there also I think writing some sort of validator class by hand and calling it is sufficient.