(v0.6.0-dev) marge v0.5.5 and use gradients on ripples.

Bringing `v0.6.0-dev` up to speed with branch `master`. I don't see any changes though; were any changes committed on the `v0.6.0-dev` branch or was it just set up to get ready?

----

Removed `docs/static/waves*` from git to save duplication as they are copied by the grunt task from `dist/`

----

Now **uses CSS3 gradients on the ripples** with the old style as a fallback for browsers without gradient support. Ripples look much more ripply now! :smiley: 

We probably need to let the user configure the gradient to their liking (or maybe we can come up with a nicer one?). If you prefer the `.waves-classic` behaviour we could make it the default and perhaps use a `.waves-gradient-ripple` modifier instead?

Feel free to revert/delete the radial gradient stuff by the way, it was just an idea and differentiates waves a little from the [polymer-button demo](https://www.polymer-project.org/components/paper-button/demo.html). :relaxed: