Include :contains pseudo-classifier

Hello Christer,

I've added support for the :contains pseudo classifier and would like for it to be included in the original package.

If I don't hear back from you in a week or so I do plan to fork this (with full attribution) so that I can publish this functionality into the maven repos. I use this library in https://github.com/GistLabs/mechanize and need contains exposed through maven dependencies.

Thanks for a great library,
@GistLabs/john-gistlabs-com