Enhancements for tweaking options of dbCursor

Hi, 

i enhanced the way of tweaking options of the DBCursor by adding additional helper namespace `monger.cursor` and changed `options`-partial, which now uses multimethod add-options.

Now, it's possible to change one or more parameter once by using Clojure datatypes.

```clojure
;;; using query DSL
(with-collection "products"
  ...
  (options {:notimeout true, :slaveok false}) ;;where keyword matches Bytes/QUERYOPTION_*
  (options [:notimeout :slaveok])
  (options com.mongodb.Bytes/QUERYOPTION_NOTIMEOUT) ;; support for previous interface
  (options :notimeout)
  ...
```

You'll notice that there is a little a bit more helper-functions than i mentioned. 
These function are useful when you need low-level access to cursor -  with that namespace, it's possible to create db-cursor, manipulate options and format output of cursor, which enables to replicate current low-level finders find/find-map/find-seq. 
That way was possible to keep low-level finders untouched.


ps: i noticed that documentation about [Querying](http://clojuremongodb.info/articles/querying.html) still misses information about tweaking query's options. I'm going to fix that issue too.