Will/tests and refactor

* Refactored `PageObject` to store a reference to `WebAppUI`, which allows pages to refer to other pages.

* Added an automated test suite for `bok_choy` with 76% coverage.  The main missing pieces are fixtures (ssh connection) and SauceLabs integration.  I included a `travis.yml` file so the tests can run in Travis.

* Added a (hopefully) cleaner mechanism for JavaScript synchronization using decorators.  You can now decorate a class like so:

         @requirejs('base/main', 'views/course')
         @js_defined('XModule')
          class MyPage(PageObject):
                 
              @wait_for_js
               def foo(self):
                   self.css_click('input')

This will ensure that `base/main` is loaded and `XModule` is defined before executing `foo`.  Classes decorated like this can also use `self.wait_for_js()` within a method to wait for all the page's requirements.

These changes should be backwards-compatible with previous versions of `bok_choy`, although the JavaScript properties in existing `PageObjects` will no longer be used.