Renamed 'nipap' module to 'backend'

The module 'nipap' used by nipapd was renamed to 'backend' due to the
unfortunate naming collision which prevented modules in the nipap
package to access the package as the nipap module had precedence over
the package.

Also, after all this messin' around, added an API function to fetch the
nipapd version.

part of #339
Added nipapd version support to pynipap

pynipap was extended with a function which fetches the version of the
currently configured NIPAP daemon.

Part of #339
Added nipapd version to WWW UI version page

Part of #339
Added nipapd version to CLI client

Lastly, also the nipap CLI client now displays the nipapd version.

Fixes #339
Merge branch 'nipapd_version'