Added --date option to the delete subcommand. You can now delete all snapshots older than a specific date.

Renamed variables for clarity

renamed variables for clarity

rename variables for clarity

fixed unclosed file error in test file

Added function for working with timedeltas from the commandline.

* had to add dateutil dependency
* changed setup.py
* minor changes to sphinx docs

Fixed some install issues

Added --date option to the delete command line subparser

renamed DateParser function

made --keep and --date mutually exclusive and required in the
commandline parser

fixed argparse_types.date_parser

* Now gives correct errors on invalid dates

Added weeks back into the date_parse pattern

Made commandline --date and --keep sepearted into their own option group
to make the requirements more clear in --help

Small changes to get tests to pass with new --date code in un-snap

Updated unsnap function to work with --date and pass tests

Added --date to unsnap_deep
Added tests for above

minor bug fixes

Created an actual launch script for use with distutils.core.setup in
setup.py

Fixed but when btrsnap was called without any options. It now displays
the help file again.

Removed debug print statement

* fixed reporting of the element entered more than once during date
parsing

* fixed delete text snapshot(s) => snapshots