Reorganized Entity Framework support

- Reorganized the code into three projects/packages:
  1) The Npgsql "core", fully functional ADO.NET and no EF
  2) Npgsql.EntityFramework, an add-on for users of EF6 and above
  3) Npgsql.EntityFrameworkLegacy, an add-on for users of EF5 and below
- All EF-related code has been split off from Npgsql and into
  the Npgsql.EntityFramework directory
- The same EF code is built via two projects: Npgsql.EntityFramework
  (defining ENTITIES6 for the new EF6), and Npgsql.EntityFrameworkLegacy
  (not defining it for pre-EF6).

Still to be done:
- Npgsql.NpgsqlFactory doesn't yet do the reflection work to return the
  pre-EF6 NpgsqlServices
- Only the 2013 sln is up to date
- nuspecs
My fix to get EdmGen/EdmGen06 and my EFv6 app working.

Tested on:
- Windows 7 Home Premium SP1 64-bit
- 3 assemblies got into gac.
  Debug-net45\Npgsql.dll
  Debug-net45\Npgsql.EntityFramework.dll
  Legacy-Debug-net45\Npgsql.EntityFrameworkLegacy.dll
- machine.config modified.
- my EFv6 app's App.config modified.
More tweaks to EntityFramework support

- Tightened some details in the reflection loading of EntityFrameworkLegacy
  (thanks kenjiuno)
- Did nuspecs entity framework projects
- Corrected SLNs for 2012, 2010