Updates to DbContext/DbSet methods per design meeting decisions

See https://github.com/aspnet/EntityFramework/wiki/Design-Meeting-Notes:-October-30,-2014#dbcontextdbset-methods

Specifically:
- Renamed Delete on DbContext to Remove
- Added Attach
- Removed async methods except for Add
- Added non-generic overloads on DbContext
- Removed XxxRange methods and added params overloads
- Changed return types to EntityEntry--see Issue #498

This is part of Issue #193