Allow generation of oauth tokens from command line

Allow generation of oauth tokens from command line

Oauth tokens can be generated for commands (scopes) having admin|user|open
policy. Restricted commands are not available as those are only usable
from ejabberdctl command line.

Four new commands are available:

$ejabberdctl oauth_issue_token "stats;get_roster"
    Generates a token authorized to call both stats and get_roster
    commands.  Note scopes must be separated by semicolon.

$ejabberdctl oauth_list_tokens
    List tokens generated from the command line, with their scope
    and expirity time.

$ejabberdctl oauth_list_scopes
    List scopes available

$ejabberdctl oauth_revoke_token "Lbs7qdJfdKXOWzVrArgyckY055tE1xnt"
    Revokes the given token