Preserve environment variables when running Popen.
Preserve environment variables when running Popen.

`Popen` does not inherit environment variables of current process
when run with `env` argument defined (and not None). We therefore
pass copy of the current environment with PYTHONPATH modified to
our needs.

This closes #69

apache/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,ChugR/qpid-proton,prestona/qpid-proton,kgiusti/qpid-proton,astitcher/qpid-proton,ssorj/qpid-proton,prestona/qpid-proton,ssorj/qpid-proton,ChugR/qpid-proton,alanconway/qpid-proton,alanconway/qpid-proton,alanconway/qpid-proton,ssorj/qpid-proton,alanconway/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,prestona/qpid-proton,ChugR/qpid-proton,astitcher/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton-j,apache/qpid-proton,prestona/qpid-proton,prestona/qpid-proton,prestona/qpid-proton,prestona/qpid-proton,kgiusti/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,kgiusti/qpid-proton,prestona/qpid-proton,prestona/qpid-proton,astitcher/qpid-proton,alanconway/qpid-proton,ChugR/qpid-proton,ChugR/qpid-proton,astitcher/qpid-proton,alanconway/qpid-proton,prestona/qpid-proton,kgiusti/qpid-proton,kgiusti/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,prestona/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton-j,apache/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,kgiusti/qpid-proton,prestona/qpid-proton