Add search to API

This is a first draft, returning a simple array with entry ID and title.

I'm missing the search function in my Android RSS reader at the moment and I think it's would be a really useful thing to have. The existing search syntax (http://tt-rss.org/redmine/projects/tt-rss/wiki/SearchSyntax) looks promising and it will be very easy to add a clean UI on Android to support searching starred/published/read/all entries with this API addition.

Please be aware that this is my first contribution to tt-rss and my first serious try to contribute something to a PHP project. I'm by no means a PHP expert (not even a beginner), so I'd be very happy if someone could pull my changes and give them a thorough review first. If there is something wrong, please comment on this pull request and I will fix whatever I broke.