Added several text editing icons

Added the following icons:
appbar_bold
appbar_font
appbar_italic
appbar_marker
appbar_textcolor
appbar_underline

The icons were made in the style of the Windows 8 Mail app icons, the ones used when making an email and selecting text.
![text icons light](https://f.cloud.github.com/assets/6247359/1818279/8321c888-6ff1-11e3-8431-08f944c879fd.png)
![text icons dark](https://f.cloud.github.com/assets/6247359/1818280/85b82dd0-6ff1-11e3-9eaf-03f9c2f12c99.png)