Adds support for distinct operation.

Unfortunately I haven't been able to write tests because I have been running
into an error, I am waiting for response from the original author of this
module.
Merge branch 'nukulb/distinct' of https://github.com/hubba/Mockgoose into hubba-nukulb/distinct
adding tests for distinct and updating distinct method
Merge branch 'master' into hubba-nukulb/count-fix-local