Changing Firefox brand+logic for WebPlatform

Squashed commit with latest addition as a single patch.

Last squash 2014-05-16 at 18:23 EST

Basics:
* This commit is meant to have "surgical" patches on top of upstream
* Currently a rebase from oauth branch

Branding:
* Adjusting README
* Use ou rown fxa-content-server-i10n
* Overriding signup marketing ad specific to Firefox/Mozilla

Account pre-existence validation form events:
* Improved extension without overwriting events hash map
* Add our own sign_up validation event handlers
* Check for existing user and add notice on the form
* Follows convention to setCustomError on form field when username is invalid
* Setting in place webservice call service
* Including shell script to check user details through PHP in scripts/getuserinfo.sh