Now Pinry will not care type of url, if it could be input, it could be
saved.
This may cause xss issue but Pinry is a private pin-board replacement,
so be it.

Allow dots in usernames

fixes #146

Also change the input type to url.

Try to optimize Docker image

To reduce the size of context sent to Docker daemon

Enable Docker Hub to build image automatically on push

Upgrade to DjangoRestframework(short as DRF) which is currently under active development support.

Summary:

    * No side effects on Pinry Browser Plugin

    * No side effects on Bookmarklet.

    * All API now working on DRF instead of tastypie.

Due to the offical warnning from command line.