Work on unit tests for multiple backends

- Unit tests now use the NUnit parameterized test suite feature
  to run multiple times on different backend versions
- Each parameterized run consults a version-specific env var
  for the conn string (e.g. NPGSQL_TEST_DB_9.1)
- If the version-specific env vars aren't defined, a compiled-in
  default connection string is used instead (simply run against
  localhost:5432). This would be the normal mode for developers.
- As discussed on the devel list, removed testing for protocol 2