[lib] Add request options to purchase call.
[lib] Check for exactly one listing.

Puchase code currently only handles one listing at a time. Add a check
to ensure this constraint.
[tools] jshint fixes.
[tools] Add purchase tool.

- Allow command line purchasing of a listing.
  - ./bin/payswarm purchase http://example.com/thing
- Uses access key authorization and authority from config file you
  specify.
- Port of examples/puchase-asset.js.