Added Ms Office docs content provider support

The idea is to use the office web apps every time someone pastes a URL that points to an office document. This is the basic implementation of that functionality.

This is how it looks like:

![excel](https://f.cloud.github.com/assets/351117/360685/6b3382ec-a1a3-11e2-9d96-ed42d436a8c4.PNG)

![pp](https://f.cloud.github.com/assets/351117/360686/72277e5a-a1a3-11e2-98d0-98d855a52bc0.PNG)