Added a few more API methods

Hi, I added a couple of new API methods: get_projects, get_filter, search and add_comment

Notice that for search to work (because the jql parameter is bound to contain = chars) we need a fixed version of Net::HTTP::SPORE, because the current version fails to escape the parameters correctly, it ought to be possible to work around the bug by escaping the parameters, but that would break once Net::HTTP::SPORE gets fixed.

I've sent the Net::HTTP::SPORE maintainer a pull request with this fix:
https://github.com/dren-dk/net-http-spore/commit/c56023391f3839998d7bb4775a54c8095a205478