Improve scc token help

[sbesson]

See http://trac.openmicroscopy.org/ome/ticket/10902

@mtbc reports:

$ scc token create
Traceback (most recent call last):
  File "/usr/local/bin/scc", line 8, in <module>
    load_entry_point('scc==0.3.4', 'console_scripts', 'scc')()
  File "/Library/Python/2.7/site-packages/scc.py", line 2195, in entry_point
    main()
  File "/Library/Python/2.7/site-packages/scc.py", line 2186, in main
    ns.func(ns)
  File "/Library/Python/2.7/site-packages/scc.py", line 1860, in create
    raise Exception("No github.user configured")
Exception: No github.user configured

It's far from clear from scc's help how to create a github.user token. A suggestion to use git config --global github.user would be helpful when this exception is thrown.