Improve cleanup option (code quality, tests, documentation)

[ehmicky]

This improves the cleanup option, but does not change its behavior:

• small code simplification
• added tests for options.detached: true (before only options.detached: false was tested)
• fix tests not properly cleaning up created processes (i.e. memory leak)
• improve option description on README

[ehmicky]

Fixed.

I've also added some tests when the child process exists before the parent process, with the cleanup and detached options. This was not tested before.

[ehmicky]

Fixed