Add feature: Health check via blueprint, default is a ping endpoint

[ppanero]

PR to solve #35

Two configuration variables are added:

• APP_HEALTH_BLUEPRINT_ENABLED to allow the user to enable the health checks.
• APP_HEALTH_BLUEPRINT to provide the import path to the function that creates the blueprint. This would allow more than one route for more than one check (e.g. uwsgi ping, elasticsearch ping, db ping).

By default (if enabled) there is only one route (/ping) that returns a 200.

A test has been provided for the default blueprint. A custom one has been tested, however, adding a test for a custom one would mean adding a views file.

[lnielsen]

@slint Is it necessary that the ping view is reachable via HTTP? I know we need it to be right now, but we could simply redirect from HAProxy instead of letting the HTTP query go all the way down to nginx.

[ppanero]

@lnielsen @slint As discussed, there is just a /ping route. By default the config parameter is set to True.

Sorry for the -f's... forgot the config file.