Store the lastid file in the users home

This patch uses Dir.home and therefore assumes that you have Ruby1.9.
Don't know if we impose this on the user so far. Also it creates a .dp2
directory in the users home and puts the lastid in there. Here we assume
that there might be more files coming, e.g. config.yml
Modify last_id storage so the user's home is used instead of cwd
Merge pull request #4 from daisy-consortium/lastid_in_home

Store the lastid file in the users home