SERVER-2033: Adding support for skip and limits within the mongoexport tool

I've added a couple of options to the mongoexport tool allowing the user to specify "--limit" and "--skip" parameters in order to grab a subset of results back from the database.

I didn't see a place to create/run tests, but I did compile a local copy with `scons tools` and played around with it for quite a bit. My team will be using this executable until these features are integrated. If there's something else needed before this can be integrated, let me know and I'll get it done. :wink:

https://jira.mongodb.org/browse/SERVER-2033
http://stackoverflow.com/questions/7631943/how-to-give-limit-in-mongoexport-in-mongodb
https://groups.google.com/forum/?fromgroups=#!topic/mongodb-user/idv5Cwpdmjk