add configurable port to command line options

A configurable port for flask enables deployment on heroku. Now the port
can be specified in the procfile.
altered error message for bad github response

There are lots of reasons the look up might fail that wouldn't prevent
the hook from being set. This new error message is more direct about
what went wrong.
Merge pull request #7 from Lonnen/configurable-port

add configurable port to command line options
Merge pull request #8 from Lonnen/error-message

altered error message for bad github response
use POST instead of GET to schedule builds on Jenkins

Recent versions of Jenkins refuse to schedule builds with GET and
admonish you in an HTML page with a 200 status code.  Oh, by the way,
improve the logging when scheduling fails.