Python libs conflicts

[XavierCLL]

To avoid python libraries' conflict between system and plugin, try to use the libraries in the system before using the libs inside the plugin. Related #67

[gena]

I'm not sure if this is a good idea, will probably result in even more errors, frequently users forget to install proper EE version globally. It feels like cross-platform QGIS and local Python environment is a non-stop source of troubles. I'd prefer if QGIS can just support packages from requirements.txt and skip global version of Python altogether, but it does not.