Rename NLastDaysTimeframe to RecentDaysTimeframe and document config
better

Should be more obvious what this timeframe (and others) do.
Change default day count to 14 for RecentDaysTimeframe

Since it's a generic version of current_month / current_week, it can be
different by default. Currently, I don't intend to have this default
(and that of other potential timeframes) configurable. Instead one
should provide explicit commandline arguments (to be added).
Revert "Drop sys.path hack"

This reverts commit 4b14c04000ed2781be33cf384842735d3981c714.

Unfortunately, I still need it from time to time and haven't yet found a
better solution.
More user-readable info in Timeframe.__str__

Using machine-generated class representations to auto-populate the
TIMEFRAME_CATALOG didn't took of, so we may as well stop (ab)using
__str__ for that. I may pick up this idea later again, maybe I'll add a
property for that instead.
Add commandline arguments (and help) to specify timeframes