[utils] - Add `pluck`, `map`, `get` and `normalize` helpers
[db-api] - Select email for proper gravatar plugin use
[models:citizen] - Use of mongoose-gravatar plugin
[law] - Map returned values with new utils helpers
[citizen] - Map JSON response with new utils helpers
[comments-view] - Use gravatar key instead of avatar
[participants-view] - Use gravatar key instead of avatar
[boot] - Use Citizen's gravatar() new method
Forgot to add mongoose-gravatar to package.json Close #203
Merge pull request #208 from DemocracyOS/mongoose-gravatar

Use of `mongoose-gravatar` plugin and first work of standarization of endpoints data exposure