Refactor server.py to use Flask #31

Hi,

I refactored server.py to use Flask.

However, I took some assertions that I hope are good but I'm not sure. You better review it and test it on not standalone mode to be sure.

- application function is replaced with application flask object (from what I understood of flask, it is the good thing to do but I didn't test it - ran only in standalone)
- post methods are now only POST (GET will be rejected)
- post data processing not entirely tested - I didn't know how to test for example comments[0] retrieved here: https://github.com/jmaher/alert_manager/blob/master/server.py#L148)
- new debug option available in config (optionnal, defauting to false) is written by default and set to true when writing conf with config.py

I hope it will be useful !
Should this be lowercase 'debug' here?