Major changes to buttons (and a new one!)

Quite a few changes under the hood.
- Buttons are fully responsive now, and will adapt to whatever font,
size or length of text put in them
- Ems and percentages rule the world now, assumably
- There's a purple button now :D

Apologies it took so long to release this. I didn't want to leave these
buttons in the dust though, since I do like them. Enjoy!