Makes wikicloth gem compatible with JRuby by using twitter-text-rb for autolinking  (with the option of using rinku on MRI).

Here's one solution that I think is reasonably elegant. This commit replaces `rinku` with the `twitter-text` [gem](https://github.com/twitter/twitter-text-rb) for autolinking, thereby removing the one dependency with native C extensions (which is causing issues on JRuby). However, wikicloth does check if it can find and load `rinku`, so if people have the `rinku` gem installed, `wikicloth` will use it (I haven't done any benchmarking, but I'm assuming rinku will be slightly faster than twitter-text). Does this seem like an acceptable solution to you?