Initial Gnus configuration.

* Modified build system so it installs Gnus configuration.
* Added user information in Gnus configuration.
Configured Gnus way of getting emails from Gmail.
Configured how Gnus send emails, using GMail's SMTP server.
Configured Gnus behaviour when explicitly saving emails.
Renamed gnus by Gnus in comments.
Configured Gnus appearance.
Configured how Gnus shows the summary and fetches old articles.
Removed two useless SMTP configuration variables.
Fixed compilation warning for Gnus configuration file.
Merge branch 'dev/gnus_config'