Browser plugin

Four main changes/additions:

* Added a ``clients/jquery`` directory with the ``$.girderBrowser()`` plugin code

* Added a test web app to demo the browser plugin

* Modified the GET() method of the Webroot class to allow manual access to the newly added "clients" directory - this was mainly so I could verify the test application was working

* Added documentation for the new plugin

Minor change to the .gitignore file as well since I am a vim person.

If there is a better name for the plugin, let's change it.

Also, I have a semi-solid plan to generalize this so it's not tied to Bootstrap.  This would be done by abstracting out the bootstrap-enabled HTML manipulation in the ``findFolder()`` and ``findItems()`` functions, replacing them with calls to user-supplied functions instead.  The Bootstrap behavior would become a specialized version of the general plugin.