Fixed _call(..) to allow user specify to jsonify or sent plaintext.
Fixed #14 all apis are now guarded by @api_guard decorator.

The guard is optional if 'key' is False or not found in the
configuration file. Otherwise, Minion backend now requires all incoming
requests to all backend must have X-Minion-Backend-Key header with a
correct backend secret key.
Removed default secret key.

We should prevent secret key being written into version control system.
By having a secret key version controlled in utils.py will encourage
users to make that mistake.

401 tests are now commented out. We need to inject the backend-key or restart
the server during the tests.
Merge branch 'issue14' of https://github.com/yeukhon/minion-backend into yeukhon-issue14

Conflicts:
	tests/functional/test_api.py