Load speed fixes from #521

These are the relevant speed fixes from https://github.com/jnunemaker/mongomapper/pull/521

I went ahead and picked related commits out into a new branch and squashed them together. This should make for more cohesive reading.

test/unit spec passes on Rails 3.2.9 under Ruby 2.0; I'm not testing the full matrix since the rspec pull request fixes the test suite across the whole test matrix.