added media files pull push feature

A lot of times I need to get the medias assets from production server to my local dev machine to tests new design, etc.

For this I added two command:

 - `cap mage:files:pull`
 - `cap mage:files:push`

Oh, and be kind, I this is my first cucumber test ;-)

Any feedback welcome.
And I've another feature (that require some cleanup) to sync database.