fix missing main/lib dir if py lib in requirements not found
remove unused listing of installed_dirs
only run pip if necessary

This commit adds and modifies a pip_guard lockfile to temp each time
install_py_libs() is called. The file is used in later runs to check
if our requirements.txt is newer. If so pip install is run if not
it's skipped. run.py -C removes the file.
style changes as requested
style update: len(line) < 80
rename guard file
Merge pull request #137 from joernhees/pip_only_when_necessary

Pip only when necessary