More generic RSS import

*Title: More generic RSS import

*Documentation:

This improvement adds support for custom RSS import definitions, and supports more fields than the earlier version.
Custom import maps can be placed in extensions.

To still use old RSS import definitions, the update script update/common/scripts/updaterssimport.php must be run.

Custom import maps must be placed in :

<extension directory>/<extension name>/rss/ez<extension name>rssimport.php
This file must contain a class of the same name, and a function called 'rssFieldDefinition'.
The function returns an RSS definition in the form of a multidimentional array.

See kernel/classes/ezrssimport.php ::rssFieldDefinition() for default RSS definition.