Move config.ini to defaults.ini, non-tracked user config in config.ini

This pull request would close issue 84: https://github.com/SSilence/selfoss/issues/84

Some people (like myself) clone selfoss right to our web servers. Tracking config.ini in git means we have to move it every time we do a pull. There's also a fairly large risk of overwriting config.ini during an ftp upload update too. 

These changes: 
1) Move config.ini to defaults.ini
2) Add config.ini to .gitignore
3) Change index.php and update.php to load defaults.ini and then config.ini. 
4) Updates documentation to reflect these changes

This allows a user to only override the setttings they want to change. As new settings are added to defaults.ini, the user will receive them with a simple "git pull" or other update. 

Eg. My config.ini now looks like this: 

[globals]
username=stuporglue
password=5d95c038a896..................2afd1
salt=12.....................................3

which overrides just those three lines from defaults.ini

Thanks,
Michael Moore