TT on multiple machines final(?) patchset

These are changes to (I hope) finish my work on running TT on two machines simultaneously. There is one change which may affect regular users: the *.bee files are now saved into the ${path} directory, instead of being written into the CSD. I believe this is an improvement in any case (it prevents stale caches being strewn around the filing system if beeminder.pl is run manually at any point) but it is a change in the existing behaviour.

The other significant change is to cache handling: a blank .bee file will now trigger regeneration of the cache, which facilitates an unusual use of the Unison file synchroniser to erase the cache rather than trying to merge it. As a consequence, it is now possible to set up a Unison synchronisation job to keep two machines' TagTime setups in sync: by using ttlogmerge as the merge program for beeminder log files and `touch NEW' as the merge program for .bee cache files, TagTime should correctly handle tags in almost all circumstances.