I came across a situation where phantomjs would not stop on a TERM
signal. I'm pretty sure it was my JavaScript that was making it hang,
but because of the sheer volume of JavaScript that had to be debugged to
find the exact problem, I found this to be more appropriate.

The default stop signal is still TERM, but now users have the option of
specifying a custom signal when asking phantomjs to terminate.

Closes #196
This was a bit confusing, there already is a pid variable (from
attr_reader). Naming is hard, I know.