Add ability to search by PID

[dylanratcliffe]

Depends on: coreos/go-systemd#385

Once the above is released, I can add the ability to search by PID. This will mean that the linkage between Port -> Process -> Service will work.

Suggested logic is that if the query cna be converted to an int, do so and then convert from there to unit name using GetUnitByPID

[dylanratcliffe]

Merged in f5a75de5182a38207b343c7126ff3eee72d94ba3

[dylanratcliffe]

Added