Add support for webfonts in themes

"Bootstrap" theme uses an outdated version of Twitter Bootstrap, and that version of Bootstrap uses images to render icons.
As on version 3.0.0 (and now 3.0.3), Bootstrap uses webfonts (and so does Font-Awesome) instead of images to render icons.

The problem was:
Stikked would not render webfonts, because they were not considered as valid theme assets.
Aside from Bootstrap, that problem would also occur if a theme used some custom @font-face targeting local webfonts

This commit introduces support for WOFF, EOT, SVG, SVGZ, TTF and OTF in theme assets.