Made the use of gravatar optional

This commit has two behaviors: 

- Makes the usage of gravatar optional, by introducing a new parameter for the site configuration. By default, this value is set to 1, active, to mimic the current behavior. The value of "0" should be used by those who don't want their instance to leak data to Gravatar.
- Modifies the return URL for Gravatar, so that it uses the same protocol as the page that is currently being served. This way, warnings about "mixed content" are avoided if the instance is being served via HTTPS. 

An additional change is to the gitignore, to ignore the "iml" file created by JetBrains IDEs (IntelliJ, PhpStorm, ...).