Modified edit_on_github.py to take path prefix, reverted matplotlib plot_directive in conf.py

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/coolprop/coolprop
Removed sphinxext, no longer needed, superceded by other extensions in IPython, MPL and sphinx itself

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Updated information on common prerequisites

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>