Big update of big things.

Help (-h flag):
	- added documentation for -d and -q flags
	- Added default value for -c and -t flags

Options:
	- Added -r flag for creating read-only snapshots
	- Added -s flag for specifying suffix instead of sequential number
	  to snapshots.

Bug fixes:
	- Instead of defaulting directory to rotate to "notset", the script
	  now exits with the intended message

Features:
	- Added function purge for doing age-based purge, for when -s flag is
	  specified. The old sequential number rotation scheme is maintained
	  when -s is not specified.