Merge pull request #546 from estolfo/bulk

Raise error when bulk execution encounters writeErrors
RUBY-823 Add read preference option to Database#command
Make NoServerAvailable a MongoError
Merge pull request #547 from estolfo/RUBY-823-pref-cmd

RUBY-823 Add read preference option to Database#command
RUBY-787 Aggregate write concern errors for legacy and write command versions
#write_concern setter on bulk ops should take in hash of options
RUBY-787 name of field is writeConcern in write command
RUBY-787 Adjusting tests and write concern handling
Update documentation for correct param type