db: add Platforms collection

Related to #85.
auth: remove salt and old hashing method

Now that we have reset-password command, we don't need this first
phase anymore. Let's use bcrypt forever and ever :-)

Related to #150.
app: added Platform type, and Platforms function

The Platform type represents a platform available for tsuru users. The
Platforms function list all availables platform.

Related to #85.
api: added handler to list available platforms

Related to #85.
api: register platformList handler

Related to #85.
cmd/tsuru: add platform-list command

This command will display a list of available platforms.

Related to #85.
cmd/tsuru: register platform-list command
cmd/tsuru: add docs for platform-list command

Related to #85.
misc/bash-completion.d/tsuru: added platform-list command

Related to #85.
cmd/tsuru: version 0.7.9

Closes #85.