Rename CommerceObjectContext to CommerceContext

Having CommerceObjectContext inheriting DbContext is inconsistent.
ObjectContext is legacy from LINQ to SQL and DbContext is from EF 5+.
More consistent would be to call it CommerceDbContext, or even more generic and library-independent - CommerceContext.
Merge branch 'master' of https://github.com/abatishchev/ploeh.github.com into abatishchev-master
Renamed remaining example of CommerceObjectContext

to CommerceContext, in order to stay consistent.