New MonoGame Logo

The team is happy to be introducing the new MonoGame logo:

![horizontallogo_128px 1](https://cloud.githubusercontent.com/assets/103522/3550203/c906f666-08d5-11e4-962d-268015c47c8e.png)

This PR replaces all the old icons, splash screens, installer images, etc with ones.

We also have moved all the MonoGame "branding" and images over to [a new repo](https://github.com/Mono-Game/MonoGame.Logo) specifically for that content.  The idea is to avoid bloating up the main development repo with source art that people rarely need.

The next steps will be updating the branding on the website, Twitter, and Facebook pages.