Removed type annotation from mongo expressions.

Figured out that the reason the type was unknown in the custom
operators is because the return type of the MongoBuilder.For member
was a fresh generic, and not IMongoQueryable<'a>.
Added support for collection/scope defined in function.

Within the quotation for the computation expression, a collection
defined on the module is represented as a static member, and hence
recognized by the PropertyGet active pattern, whereas within another
let definition, is embedded directly as its Value.

Refer to http://stackoverflow.com/questions/18413138.