(WIP) RTL Sass compilation

Hi @nedbat and @frrrances,

I'm trying in this PR to load the correct layout direction (RTL or LTR) for the users based on their language preference. This is based on @frrrances RTL work #2682. See my [original comment](https://github.com/edx/edx-platform/pull/5309#issuecomment-56484498) for more info.

**Done:**

  - Compile two versions of the Sass into two directories `sass` and `sass-rtl`.
  - `ENABLE_RTL` feature to turn on and off the compilation. This should save compilation time for LTR only platforms setups.

**TODO's:**
 
 - Create a new Pipeline finder to load the correct compiled CSS files

Please tell me what do you think?