chop.py
  - incorporated argparse module
  - now using Streams object
  - Added usage to cmd line help
filter.py
  - fixed help
responses.py
  - tabs=>whitespace
streams.py
  - tabs => whitespace
  - write() method fixed - traces don't always have encoding set.
  - plot() method now can write to a file non-interactively.
           -
Updated sensorloc url
Added type to argument parser for utcdatetime, removed empty function in test script
start/end now can be None by default, and fixed help text for those options
Merge pull request #1 from mhearne-usgs/refactor_chop

Chop appears to work as described, merging.

Some comments to consider for the future:

When running without arguments, I think the usage parameter to argparse suppresses the default help output (so you have to run with -h).

The tests appear to need to be run manually.  It would be nice if they provided output saying pass/fail, etc.