refactor(client): Convert jQuery AJAX promises to Promises A+ promises.

jQuery AJAX promises are not Promises A+ compatible and can cause unexpected behavior. Monkey patch p to add a jQueryXHR function, and replace all references to 'p-promise' with 'lib/promise'.

fixes #1406
Merge pull request #1407 from mozilla/issue-1406-jquery-promise

refactor(client): Convert jQuery AJAX promises to Promises A+ promises.