Make the Page look more flatter and modern.

These are some things that bothered me about the page, and I wanted to fix them. 
What I've done so far is make the buttons and navbar flat. What I plan to do is also change colors of some elements to green, because that is the color of LMMS.

I opened this PR to get feedback from you guys, so opinions are welcome :)

Screenshots: 

Main page

![main page](https://cloud.githubusercontent.com/assets/6282045/5403583/80a613ca-818e-11e4-9530-42dd70a5ce61.PNG)

Community page

![community page](https://cloud.githubusercontent.com/assets/6282045/5403638/8b80ecac-818e-11e4-9f96-e0a56785046b.PNG)

Downloads page

![download page](https://cloud.githubusercontent.com/assets/6282045/5403646/96dd9276-818e-11e4-95e3-3c1f4fe7959b.png)

I'd recommend keeping `btn-default` and just override its style in our custom style sheet.