split #insert, refactor send_write, remove/delete with :limit

RUBY-676
Documentation for bulk write API and #remove with :limit opt

RUBY-676
#insert optim with respond_to? plus documentation tweak

RUBY-676
minor: adding MONGODB qualifier to all auth test env vars
RUBY-648 adding support for x509 auth mechanism
RUBY-648 adding tests for x509 auth mechanism