Added mediarestore command

[ZuluPro]

I added a mediarestore command and upgrade all related code:

• Tar file was made with absolute path from root, so untaring had to be made to root for get files in right place. It didn't allow to restore files in other address. I changed it for include media files in a directory named 'media'.
• Instead of archive MEDIA_ROOT, the media storage object is used for open files.
• Files are now read 1 by 1 from storage and write into tar file and reverse, it allows to create (in future) exclusions (PR Exclude some media directories option #45)
• I also tried to make this process as efficient as possible, so I change mediabackup for use spooled temporary file

[coveralls]

Coverage increased (+0.9%) to 73.279% when pulling 1a91211 on ZuluPro:mediarestore into 94b7848 on django-dbbackup:master.

[coveralls]

Coverage increased (+1.7%) to 74.093% when pulling 0c38e72 on ZuluPro:mediarestore into 94b7848 on django-dbbackup:master.

[coveralls]

Coverage increased (+1.7%) to 74.15% when pulling 51e8529 on ZuluPro:mediarestore into b7c63a8 on django-dbbackup:master.

[ZuluPro]

I think all is great, @benjaoming @mjs7231 ?

[benjaoming]

Looks good - with tests and everything.

Wondering, all the CommandError: There's no backup file available. outputs in the tests, are they anticipated?It's in themediarestore` functional.sh tests as I read it..