Add documentation about all Env variable usage

Errbit can be config anly by ENV variable. We need a complete
documentation about it and improve this variable capabilities with not
only Heroku point of view
Improve information about ENV configuration
Fix spec because now the spec use TimeZone too in Mongoid
Add link to the ENV documentation from the README
Merge pull request #507 from shingara/features/improve_documentation_about_variable

[WIP] Features/improve documentation about env variable