Add a function to get raw base64 content on Base64Fields.

Base64Field now provides a get_<fieldname>_base64() function that will
return the actual Base64 content, rather than the decoded content.

Reviewed at https://reviews.reviewboard.org/r/6209/
Backport splitting all release notes into ReST files.

On release-0.8.x, we moved from the NEWS file to individual ReST files
for the release notes. This backports this change to release-0.7.x.
Require Django 1.4.14 or higher.

Django released a new security update today, which fixes 4 significant
security bugs. We now require this release at a minimum.
Merge branch 'release-0.7.x' into release-0.8.x