Improved backend UI elements.

### Summary

I have updated the sidebar and some form element style. I believe this is a small step forward to make Sylius more polish, less bootstrap.

This also include some other minor UI fixes.

Before:
![screen shot 2013-11-23 at 6 33 30 pm](https://f.cloud.github.com/assets/193112/1606314/de85703c-542e-11e3-9eee-cf117e877adf.png)

After:
![screen shot 2013-11-23 at 6 44 17 pm](https://f.cloud.github.com/assets/193112/1606315/e3627262-542e-11e3-8ac2-a5304b097559.png)

### Details

* Sidebar: Small font size to avoid word wrapping.
* Sidebar: All cap but smaller font in sidebar section header.
* Table: Now includes a very light background. 
* Table: Lighter border.
* Icons: Removed Font Awesome and use Glyphicon instead as it is the icon font that comes with Bootstrap 3.
* Form elements: Thick border to match Sylius's bit font design.
* Buttons: Use Glyphicon as well.
* Buttons: Remove rounded corner.
* Buttons: Thick border, easier to tell which one is label and which one is button. Also match with text box style.
* Paginator: Update style.