Added Google Web Font support

Designers can now use Google Web Fonts in their emails presuming the recipient email client supports them (typically only mobile platforms).  Google Web Fonts are enabled by including one or more font style URLs to the project's config.yml file:

fonts:
 - http://fonts.googleapis.com/css?family=Open+Sans:400,700

Then simply add the family name to the appropriate font or style-able element (e.g. td, div, etc.).  For example:

{td font-family="'Open Sans', sans" font-weight=700}

This works by embedding the CSS provided by Google from the URL specified into the <head> of the rendered email.