chore(API): add ngConstant to manage dynamic URL in Factories
chore(heroku): finish configuration for Heroku deployment

- Add ngconstant to setup API URL depending of the environment
- Update Gruntfile and configure ngconstant task + register it
- Add config.js as dependentie to app.js + factories to be able to use API_BASE_URL constant
chore(Heroku): add Morgan middleware to log requests