This new version exports the features as directories and no longer require to
have to modify the settings to explore another feature. It is also
significantly faster since we no longer make one API call per image.

We also leverage the python-500px library, instead of using urllib. A patched
version of this library is embeded since the upstream version does not
support array arguments yet.
Add paginated views.

Add a "Next page" entry when the number of images in the current collection is greater
than the number of items displayed per page.
Add search mode.
Allow to browse by category.
Better dir names
Fix broken default value.

If rpp is 0 then 500px servers return a 500 error code.
Rename quantity setting to rpp to avoid conflicts with previous settings.

In v0.1 the quantity setting was an enum. We cannot safely reuse it for a
labelenum in V0.2.
Merge pull request #1 from cykl/more_features

More features