allow multiple write options including wtimeout

[rednar]

Current options only allow one single write option. Create a own Optionable object to check write option hash. Also include 'wtimeout' to allowed write options.

[leemhenson]

:+1: it would be great to get this one merged in....

[arthurnn]

we will need tests for the new option.

[leemhenson]

@arthurnn see https://github.com/mongoid/moped/pull/355