Use grunt-webfont for icons

This should make it easier to extend SirTrevor's custom web font with new icons.
It uses [grunt-webfont](https://github.com/sapegin/grunt-webfont) to create a CSS file with the embedded font during the grunt default-task.
I had to make a small adjustment to the Twitter-Block, as the `data-icon`-Attribute was wrongfully entered as `twitter`, and not `tweet`.

Currently there are still some issues remaining:
- For some reason, the symbols in the Block-UI buttons are way to small, I haven't yet figured out why
- The `tick` and `close` source SVGs are missing from the repository