fix: correct methods overriding

Don't add new listener for kill event, but override ProcessLauncher's methods for correct exit.
fix: Allow passing props to WebDriver

Fixes #4
Closes #11
fix: Default spec values

The default values are not being applied because they are not defined on
the args.

Related #11
Merge remote-tracking branch 'j0tunn/fix-hang-after-kill'

Conflicts:
	index.js