Add terms on client side.

I know that this should be proposed as a patch in mozilla-central.
I put it here to work on the patch and then I will make the appropriate patch to mozcentral.

 - [x] HTML / Ajax term loader based on the Accept Language of the browser (given back by loop-server)
 - [x] Extract ToS from legal-docs using grunt and bower.
Do we need to add these as well? (they seem to be generated files)
-	content/legal/styles/main.css
-	content/legal/terms/en_US.html

```sh
$ git clone git@github.com:mozilla/loop-client.git --branch 1044411-legal-terms
$ cd loop-client
$ make install
$ git status
On branch 1044411-legal-terms
Your branch is up-to-date with 'origin/1044411-legal-terms'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

	content/legal/styles/main.css
	content/legal/terms/en_US.html
```