Add JSON output with new --json/-J option

[jasoncdavis]

Added support for a -J output flag to report ping results in JSON records - this is more desirable for programmatic consumption of the fping data

[jasoncdavis]

Added support for a -J output flag to report ping results in JSON records - this is more desirable for programmatic consumption of the fping data

[coveralls]

Coverage decreased (-1.3%) to 78.402% when pulling cb09613 on jasoncdavis:master into f9e1964 on schweikert:master.

[schweikert]

Thanks for your contribution! There are some changes that I would do before integrating it:

• Call the option --json, instead of --JSON for consistency with the other options
• Use standard output for the JSON output when in JSON mode, and stderr for everything else (as it should be also for the non-json mode, but we can't fix it there anymore)
• Add some tests for this

[schweikert]

Continued in #160