[WIP] Flat design

In the pull request #4, we already have enhanced the global design, with a full width design that includes a top navbar, a left menu and a wide body. This design is responsive with three steps (mobile, tablets and desktop).

In this PR, I have worked on cleaning the design in order to introduce a flat design.
An overview:
![flat desktop](https://cloud.githubusercontent.com/assets/1638158/3421979/30f27754-ff0e-11e3-87bb-058bbae344a2.png)

The mobile view:
![flat mobile](https://cloud.githubusercontent.com/assets/1638158/3421982/4032a0d6-ff0e-11e3-8e10-2e2be2a9b7d7.png)

This new design will be included in [CentralReport](https://github.com/CentralReport/CentralReport) with a refactored web server.

So, what do you think about this?
(cc @Ninir)