Fix reset in live examples (on natureofcode.com)

- Dispose previously running sketch before starting one when `RESET` is pressed.
- Update `PAUSE` button label according state (i.e. `RESUME` when is paused, `PAUSE` when is not).

Fix shiffman/The-Nature-of-Code#25
Merge pull request #26 from mrfabbri/master

Fix reset in live examples (on natureofcode.com)