Pipeline tool help improvements(on Windows).

Changes : 
-Help button now opens "http://www.monogame.net/documentation/?page=Pipeline" instead of just documentation.
-About button must opens about dialog box instead of webpage(because its a behaviour intended for all windows desktop apps), so I created this dialog. I think its cool.

![image](https://cloud.githubusercontent.com/assets/3036176/5535970/4917077e-8a9a-11e4-868a-19b92ab4ae66.png)