Workaround for mono .NET 4.5 + ClaimsIdentity

Made sure to never have the function accessing the
WindowsIdentity type JITted under mono .NET 4.5.

https://github.com/npgsql/Npgsql/issues/133
Merge pull request #134 from roji/133

Workaround for mono .NET 4.5 + ClaimsIdentity