Correct GetConstraints and GetConstraintColumns : use of "as" keyword to...

Hi @kenjiuno,

I tested your pull request (https://github.com/npgsql/Npgsql/pull/213/commits) over the ngsql master branch. This works great !

My configuration : 
   Visual Studio 2003 Update 2 RC
   Visual Studio SDK
   Postgresql 8.1.10 under Linux

To make it work with my version of postgresql, i had to patch GetConstraints and GetConstraintColumns in npgsqlSchema.cs (use of "as" SQL keyword).