Only bind file upload callback for image assets

SectionAssetEditView.processForm was binding the image uploading handler
for all asset types, not just images.  This is wrong, so I added a guard
to check for the existance of an 'image' field on the form.  This seemed
more appropriate than checking the model type.
Bump Solr version in Travis build

Bump the installed version of Solr in the Fabric task from 3.6 to 3.6.2.
The 3.6.0 version is no longer hosted at the URL being used to download
the package for the build, causing the tests to fail in Travis.
Update position and behavior of builder layout selector. Also add "Layout" label. Addresses #442.
Merge pull request #577 from patternleaf/442-layout-selector

Update position and behavior of builder layout selector.
Scale latest item images

Use the max-width CSS property to scale externally-hosted images that
we're not able to scale server-side.
Change the story license immediately

In the publish/share view of the builder, the story license (and the
Creative Commons image and link) change as soon as the radio buttons are
changed, eliminating the need for a submit button.

In order to make this work, the Story needs to have a default license value.
The Story model has been updated so that all stories have a 'CC BY' license
when instantiated.

I also removed the padding on the forms in the Publish/Share view, made the
intermediate text (the text shown before the CC image and link are loaded)
indicate that more data is being loaded, and added a fallback message for
when the request to the CC API fails.

Closes #517.
Merge branch 'develop' into 35-upgrade_to_django_1.4