NinjaServer merge, resource finding improvement and version explicitness

Hi Henri,

I've made some changes to how Nagstamon searches for the resources directory where pkg_config fails (or is missing). The changes should be portable enough.

I've also made a small change to the NinjaServer, by merging the inheriting Ninja2 and Ninja3 servers since they are the same apart from a single element find call. I think this will be an improvement for Ninja users in that they can be agnostic as to which version they're on and everything will Just Work (tm).

This set also contains a change to the shebang of nagstamon.py that adds explicitness about the python version we require (i.e version 2).

Let me know if you feel something's amiss!

Best regards,
Anton Löfgren, op5