Added support for the queries of similar questions (categorized as a "search" method to mirror the organization of the API docs)

[chrisvfritz]

This is the query that is now supported through RubyStackoverflow.similar('This is the title of a hypothetical question', { addition_option_1: 'foo', additional_option_2: 'bar' })

I also improved URL parameter parsing, so that strings with spaces could be accepted into the title and not result in an invalid URL.

[chrisvfritz]

I also added support for the /tags query. I see that the latest Travis CI build failed, but it should pass if you restart it. The build failed due to a temporary problem accessing GitHub, rather than anything to do with the code.

[raysrashmi]

@chrisvfritz thanks for the pull request. can you please add spec