Remove list item bullet

Remove list item bullet in Select2 inputs on tag step in builder.
Only send non-null values to server

When sending a PUT request to the API, using multipart/form-data,
only send non-null model attributes. Otherwise, Firefox seems to encode
the null values as strings with a value of "null" instead of empty strings.
This seems to confuse the server-side code.

Addresses #738
Support asset image replacement in IE9

In IE9 (and lower) we have to send data in a form inside a hidden iframe
element. The HTML spec doesn't support using the PUT method in a form, so
we need to implement ``AssetResource.post_detail`` so the server will
accept the request.

Since this breaks the semantics, at least as Tastypie defines them, of a
POST to the detail endpoint, we should only implement this if the ``iframe``
querystring parameter is present.

I refactored some of the code in ``AssetResourceTest`` to avoid repeating
code in the tests for this fix.

Closes #749, addresses #738