Change default img icons to responsive font icons

For master branch, maybe require version bump.

Generated from fontello.com
Using Entypo fonts by Daniel Bruce
Licenced under CC BY-SA

Using embedded fonts for minimal approach.
Tweaked icon names to avoid collision.
Set to 280% font size per default.
Support CSS for IE7 has been skipped.

Only includes normal and circled (__default__) icons for:
- Facebook
- Github
- Google+ for Google Oauth2
- Twitter

Note: The normal style looks unbalanced but are good for additional styling since they are free from fixed forms. The circled style is set as default since its most out-of-the-box shaped.

![old-style](https://f.cloud.github.com/assets/66969/2178658/7efe3bd6-9672-11e3-9284-9e4e823b1fff.jpg)

![new-cercled](https://f.cloud.github.com/assets/66969/2178659/901b7e7e-9672-11e3-8d89-b70f0ce48f17.jpg)

![new-non-circled](https://f.cloud.github.com/assets/66969/2178661/96b0aa66-9672-11e3-8151-6c7d945f6a90.jpg)