Add support for save changes async for issue #1

[togakangaroo]

Not sure what to do with EfHooks/EfHooks.Test (the non .Net 4.5 version). Since < 4.5 doesn't support the async keyword that project no longer compiles

Issue #1

[togakangaroo]

I tried to rewrite it without the await keyword but that's not going to fly since the 4.0 version of EntityFramework doesn't actually have a SaveChangesAsync to overwrite. Therefore the current strategy of using the same HookedDbContext file and compiling it in two different projects fails for the 4.0 version.

Possible solutions

• Up the major revision number and drop support for .Net 4.0. This is what I did in this PR and frankly do you really want to support all old versions of .Net going forward?
• Use something like t4 templates to generate HookedDbContext
• Extract all common code that doesn't reference EntityFramework into a new "Core" project and then have each EFHooks project delegate responsibility for everything to a class in the Core. This means an extra dll for everything but is technically the neatest option.

My vote is still to drop support for old versions of .Net

[visoft]

I'm up for dropping .NET 4 support at this point. EF6 for 4.5 is a different beast and should just be rewritten as a 4.5 assembly. For legacy support, users can keep using the current version for those using EF6 with .NET 4.

I haven't had a chance to review your code, but I think we can probably do a bunch of cleanup since we won't have .NET 4 support anymore.