Simplify installation & various minor fixes

[valgur]

This PR suggests a slightly simpler way to install and run the plugin scripts by getting rid of the need for lines like

echo -e "baseLoc='${cwd}/'\n$(cat colorize.py)" > colorize.py

in the installation script.

And a few other minor things:

• Merges the Ubuntu and Mac OS install scripts (although I have not tested it on the latter).
• Moves requirements to requirements.txt, so they are easier to manage.
• Fixes a bug in extracting the weight files for deeplab-v3. (Fixes #2, fixes #9 and helps with #3)
• Moves the gimpenv paths from the end of sys.path to the beginning, so the gimpenv packages are preferred over the system ones. This fixed an incompatibility between scikit-image and numpy versions in the pythonenv and the system for me. (Also possibly fixes #3 for some)
• Avoids adding gimpenv to sys.path repeatedly.
• Removes invert.py, which I assume was a test plugin not meant to be included.
• Adds .gitignore and removes all *.pyc files.
• Adds gimp-python to Ubuntu dependencies. (Fixes #3)

[kritiksoman]

Sorry we were not accepting PR as GIMP was transitioning to python3 and python2 was already deprecated. If you are interested, please contribute to this branch which is meant for python 3.