Added search_tweets tag, and urlize_twitter filter

Hi,

I've used your twitter-tag project with great enthusiasm in a project of mine.
I also needed some enhancements to the library, like searching for tweets instead of listing the user feed.
They are included in this pull-request. I hope you can include it in the official version, and soon release it as a new version so everyone benefits! (and I can remove a github requirement line.. ;-)

Greetings,
Diederik