first work on geoSearch
Added geoSearch support for models

All tests passing and geoSearch works. Still need to cast the return
documents back into their models
Added casting/lean support for geoSearch

also, updated the tests. everything now casts appropriately.
small fixes
fixed an issue that could have caused callback to be invoked multiple times, now passing errors more node-like
updated the tests to work with new error style
forgot to remove an only()
fixing up how errors are sent back to user

making sure that errors sent back to the user are `instanceof Error`
fixing typos
Tests corrected

updated the tests to reflect the new way the errors are passed to the
user
Merge pull request #1560 from 10gen-interns/geosearch

add model.geoSearch