Add admin authorization system to new users

[leportella]

This pull request closes all tasks on #19 and #20

• Create a new endpoint for admin
• Verify admin privileges to access new endpoint
• Add list of users on the system
• Allow to authorize users that had signed up to enter the system
• Make users created by signup unable to access the system until admin authorization
• Verify authorization of admin when users try to authenticate to the system
• Admin users defined on the config file should be created in SignUp but they are already authorized to enter the system

The Authorization area looks like this:

[yuvipanda]

This looks awesome!