admin_token identity support

adds basic admin_token support to jumpgate whereupon a shared secret
(the admin_token) can be used to authn/z to the jumpgate API. this
secret does not map into the SLAPI.

implements https://github.com/softlayer/jumpgate/issues/35
admin_token support

updates in-line comment for clarity.
Merge pull request #36 from bodenr/features/admin_token

admin_token identity support