[Fix #69] Support the flask APPLICATION_ROOT config

[deckar01]

Prefix the formatted swagger path with the APPLICATION_ROOT from the flask app's configuration settings.

I mentioned in a comment on the issue that I preferred to lean on the url_adapter interface, but it does not expose a string transformation of the endpoint that does not require providing the values for the parameters.

Note: I opted for the try/except import to support python 2 & 3, because future is not currently a dependency of the project.
https://docs.python.org/3/howto/pyporting.html

[sloria]

Looks great! Thanks @deckar01