Release 0.19.0

[northernSage]

Our 0.19.0 release 🎉

Change summary

Mostly internal changes. pytest-xprocess is now a package in order to avoid issues such as #77 and also, quoting from #85:

We now have a XProcessResources class where all resources used by a running process will be kept. Each XProcessResources knows how to clean itself up by using its release method and will do so upon exit under normal circumstances through its destructor __del__. Upon any interruptions during the test run, the clean-up process will be forced by calling XProcess._force_clean_up in our InterruptionHandler

[northernSage]

postponed for now, see #98