Export - allow export of PRNG state from any object that has one

[GoogleCodeExporter]

The randomisations currently support the export of the PRNG state (initial and 
current).  

It would be useful to generalise this so any object that has PRNG states can 
export them.  This makes replication easier.  

It should largely be a case of moving the export subs out of 
Biodiverse::Randomisation to Biodiverse::Common.  

The next step will be to add validity handlers in the event there is no PRNG 
state (e.g. for Biodiverse::Spatial objects).

Original issue reported on code.google.com by shawnlaffan on 31 Jul 2012 at 3:34