Fix doc string link
Merge pull request #138 from rozza/3.0.x

Fix doc string link
Implemented DropCollectionOperation, after much heartache and a false where I should have true-ed.
DropCollectionOperation now working with changes from 3.0.  Tests added.
Removed unused fields/whitespace
Implemented DropIndexOperation for driver and driver-compat; deleted DropIndex command; moved the basic dropIndex tests from the old collection test to the new functional test; added more tests for dropIndex in the driver module.
Fixed results of codereview:
 - Constructor params on DropCollectionOperation
 - Missing executeOperation on DBCollection
Also added JavaDoc in places.