Postgis types

I've started to implement the most common geometric features of Postgres Spatial Extension as Npgsql Types to easily manage them in npgsql commands. I use it in some private projects, but this is always work in progress. It will be great to have some feedback.

Thanks !

[addendum] Code tested with .NET 4.5 and PostgreSQL 9.1 + Postgis 2.0
Rather than return 0.0.0.0, it's a better API practice to return null for PostGis not installed