first group of changes for USPS tracking

Start of adding USPS tracking capability

* Currently using FedEx sample doc; needs replaced
* Unit only - not remote yet

passing unit tests for USPS tracking

add USPS timestamp tracking info

further development of USPS tracking

USPS tracking response working

To-do: parse tracking events for location, datetime, message

Cleanup, additional testing, parsing of more USPS data

Handle package not found in the test environment

general cleanup prior to pull request

more general cleanup prior to pull request

further cleanup prior to pull request

Address date-format unit test failure in 1.8.7

Add carrier name to USPS tracking response
Gets tests passing
Merge branch 'pullrequests/usps_tracking'
Fixes link to travis-ci