better filtering on `Content-Type`

This patch includes more checks for acceptable `Content-Type`s, namely XHRs and Fonts.

On a side note, I wonder what are the criteria in Chrome's dev tools for Fonts and WebSockets. (I've been trying to think of a clever way to get all the `@font-face`s used and compare it against the list of resources in my phantomjs output.)

P.S. Awesome tool - can't believe I didn't find this sooner!