Update to my CodeIgniter MongoDB library

I’ve updated my CodeIgniter MongoDB library to version 0.5.1 (zip tar.gz spark). It mostly contains bug fixes and optimisations.

Version 2.0 of the library is almost finished - check it out at https://github.com/alexbilbie/codeigniter-mongodb-library/tree/v2. I’m currently working out how PHP Unit works so that the library can be tested on Travis CI each time a new commit is made.