Enable building Text indexes and setting weight options

NOTE: Text indexes are only available in Mongo 2.3 / 2.4

Conflicts:
	MongoDB.Driver/Builders/IndexKeysBuilder.cs
	MongoDB.Driver/Builders/IndexOptionsBuilder.cs
CSHARP-874: added support for setting weights on a text index.
addressing code review comments.
CSHARP-613: added test proving that this is fixed.