Improve project home page usability

I looked at project page and saw next problems with it:

* a lot of information but not separated on blocks - hard to understand
* unclear how to contribute to project

Instead of complete redesign I followed step-by-step changes.

### Step 1

First of all I separated Repository and Contribution blocks.
Contribution block should makes it easy for people to contribute. Even when we visit GitHub project page its not clear how to start. Our Contribution section should help with it. All repository stats and download button moved to Repository block.

![Screenshot_2015-05-05_11.49.16](https://gitlab.com/gitlab-org/gitlab-ce/uploads/e69977603acfdc381acb539b19e8cba1/Screenshot_2015-05-05_11.49.16.png)

### Step 2

Next step was to make top section less loaded. I moved clone panel to sidebar because it is very heavy element and rarely used. If you use web editor - you dont even use it. If you work via http/ssh you will copy remote only once. So my idea - it should be on home page but should not be in center of attention.

![Screenshot_2015-05-05_12.47.04](https://gitlab.com/gitlab-org/gitlab-ce/uploads/41633cc8915bb12c979230567b40aea4/Screenshot_2015-05-05_12.47.04.png)

See merge request !610