add --httpLogging option to log HTTP requests and responses

The option will log the HTTP request and response into a file called
http.log on validation failures along with some error details. This can
also be set in a testng.xml file using

    <parameter name="httpLogging" value="true" />

This feature requires we move from rest-assured 2.3.0 to 2.3.1.

Fixes #156
Merge pull request #159 from spadgett/master

add --httpLogging option to log HTTP requests and responses