CSHARP-692: Add support for the $comment meta operator.

This allows people to attach comments to cursors, which will show up in
the system.profile collection during profiling.
CSHARP-692: Add .WithComment() method to the LINQ Provider.

This allows you to send the $comment meta operator with a LINQ query,
so it wil be recorded in the system.profile collection if profiling is
turned on and the query is slow.

Usage:

var result = collection.AsQueryable()
                       .Where(o => o.Field == 2)
		       .WithComment("Hi there")
		       .ToList();