BUGFIX: CVS Import with Zero-Values (0)

Corrected bug to allow the import of 0 values from CVS files.
Detailed description of the bug:
http://forum.nconf.org/viewtopic.php?f=17&t=1186
Added check_command handling for service-templates

Added the special check_command handling for service-templates.
This allows the extension of the service-template class with
the check_command attribute.
Import hostgroup with realtionship to advanced-services

Allow to import hostgroups via CSV file and connect the hostgroups to
the respective advanced-services.

Example CSV-file:

hostgroup_name;alias;advanced-services
hostgroup1;First hostgroup;ping_service