Reuse running adb instead of killing existing one
Accidentally kept typo in ps command

It was there in the first place to test the behavior if `ps` does not
exist somewhere in PATH
Elaborated on addon-sdk confusion over `require`
Calling ADB.kill resets didRunInitially
Fixed push-to-device problem

There was a `self` reference when there should have been a `this`
Merge pull request #584 from bkase/adb-noclobber

Reuse running adb instead of killing existing one. Fixes #464