Provide a path to python-requirements.txt w/ installing Python deps

[IamPhytan]

This PR is a fix to #100.

When installing Python modules, this PR adds a verification to the process:

• If ${KIRI_HOME} is set and if ${KIRI_HOME}/python-requirements.txt exists, we install Python modules from the local python-requirements.txt file.
• Else, we install the modules that are listed in the raw python-requirements.txt file on the GitHub repo

Before merging, I would check with @MarcelRobitaille, as this fix may hinder on the build process for the AUR package he maintains, as per #82.

[leoheck]

Thanks for your PR. I changed it a bit. If KIRI_HOME is empty then it sets it with "./" as it was run locally for the first time.