Rewrite CriteriaHash#merge to be less gnarly. Add $or merging.

This add collapsing of multiple $ors or $ors + $ands containing $ors into a compound $and statement.

* Closes jnunemaker/mongomapper#298
* Closes jnunemaker/mongomapper#499
Merge pull request #36 from cheald/master

Fix issues with OptionsKeyNormalizer acting on sort fields
Merge pull request #37 from cheald/compound_or

Rewrite CriteriaHash#merge to be less gnarly. Add $or merging.
Release 0.6.5.
Fix sort key mappings. Closes #38.
Release 0.6.6