Remove 'http:' in `src` attr of `img` tag.

Imgur removed the 'http:' in the `src` attributes of the `img` tags in
albums so it's now "protocol relative", and use the protocol used in
the request.  This broke `imguralbum.py`, though.

This commit removes the 'http:' in the `findall` matcher of album
images and adds it when downloading the images, and makes
`imguralbum.py` work again.
Add support for https.

Adds the https protocol to the imgur url validator, and also downloads
images using https if the album url parameter uses https.
Merge pull request #4 from lemuelf/protocol_relative

Protocol relative and HTTPS support