Setting $? in Kernel#`

although with stubs in place for Process::Status#stopped? and
Process::Status#exited?
Using the existing IO#pid accessor
Unstubbing and doing as JRuby does

for `Process::Status#exited?` and `Process::Status#stopped?`.
NoMethodError is kinda like NotImplementedError, right?
Merge pull request #548 from modcloth/mbh/backtick-process-status

Kernel#` sets $?