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