separated kashif's changes from original functionality, added initial grid-mapfile support

Kashif put too much functionality into the original module. There was not enough common code to justify this move. I've separated both ways of pool account generation and init.pp just calls the desired method.
Kashif needs to check if it still works for him. I've had some problems to get the users.conf parsing to work properly with our configuration file. I have no interest in using the users.conf parsing, so I'll not pursue that any further.
Added a define to create grid-mapfiles and groupmapfiles (initial version).
Added a simple wrapper class for prefix based pool account creation which allows to define the pool accounts in hiera. This is currently broken because of Kashif's changes.