Birch theming demo

Howto

Run the virtual machine using vagrant up.
Change colors using the _colors script, using it as follows:
```
  ./_colors red blue
  ./_colors '#3ea5ce' '#1e9ed7'
```
Revert to the default theme with the _revert script as such:
```
  ./_revert
```
Change images by opening the images symlink, and replace the logo-open-edx.png (desktops and tablets) and logo-edx.png (small devices).