Add a :max_results option to JIRA::Resource::Issue.jql search

I've found jira-ruby fulfils all my needs except this one tiny piece of missing functionality, so I thought I'd put together a pull request for it. I've copied the approach used by Jiralicious, which is to [accept an optional `options` hash](https://github.com/jstewart/jiralicious/blob/07d074b05d5d039797997dfff9d4de282e648aa8/lib/jiralicious/search.rb#L7-L9) as a catch-all for JIRA's various extra search parameters.

Please let me know if this isn't good enough to merge. I'll be happy to iterate on it until you're happy with it! :)