Add support for registered classes in BsonClassMap

[ircnelson]

Would be interesting look into BsonClassMap and get mapping informations instead of using Attribute, because many times we have isolated entities/pocos in determined project and can't add references for one or more technology.

Other problem is: sometimes we make routines thats use reflections to generate map dynamiclly on BsonClassMap.

What you think about this?

[crhairr]

This is a very good suggestion. I'd always thought of the DbContext as the initiator for creating the model, but there's nothing wrong with tapping into OnModelCreating and generating EntityTypes from BsonClassMaps. After all, BsonClassMap registration is (infuriatingly & untestably) static, so they'll always be created once per app domain, which could make generating the model from them a bit faster.

I'll add this to my list updates. Thanks for the suggestion!