Support for ReadPreferences on (fetch ...)

Added:
- A function to create a ReadPreferences instance (read-preferences ...) (fixes #109)
- Functions to get/set ReadPreferences on a per collection basis
- Functions  to get/set WriteConcerns on a per collection basis

Changed:
- (fetch ..) now creates a DBCursor instance via constructor
- (fetch ..) now accepts :sort and :one? (because the DBCollection does)