Numerous PEP8 style fixes and removal of unused imports.

--HG--
branch : PEP8
Merge.

--HG--
branch : per_site
Optionally allow the media library to be segregated on a per site basis, configurable via a Mezzanien editable setting.

--HG--
branch : per_site
Replaced a few more instances of DIRECTORY.  Cleaned up comments.

--HG--
branch : per_site
Merged in joshcartme/filebrowser-safe/PEP8 (pull request #5)

Numerous PEP8 style fixes and removal of unused imports.
Merged in joshcartme/filebrowser-safe/per_site (pull request #6)

Media library segragation based on site
Fix dir paths for MEDIA_LIBRARY_PER_SITE.
Move root dir creation to new get_directory().
Move MEDIA_LIBRARY_PER_SITE to Mezzanine so it will be documented.
Don't assume Mezzanine has MEDIA_LIBRARY_PER_SITE yet.
Use storages API for dir creation.
Version bump to 0.2.15
Remove debugging
Better support for S3.
Version bump to 0.2.16
Hide the rename and delete columns in the generic popup

The popup is meant for browsing and selection, not editing and library
management. Renaming and deleting submits forms and confuses the
parent page in which the popup dialog is opened.
Handle links correctly inside the generic popup (jquery dialog)

Specifically:

- If target="_blank", let it be handled normally (new window) instead
  of opening it inside the dialog. Fixes image links.

- If href is a just a query string, append it to the current url that
  is open inside the popup instead of the parent page url. Fixes
  filters and sort links.
Hide the upload and new folder buttons in the generic popup

The buttons require scripts that are not available in the popup,
because the jquery .load() function strips out <script> tags.
Apply popup layout tweaks to all popup types.
Version bump to 0.2.17