Fix path-dependency issues with make commands

Forced a cd to the directory containing the file being compiled prior to
running make, Prcompile, and Prdeploy commands. Without this change, the
file will not correctly reload after building unless Vim's working
directory happened to be the right one already.
Add README and GUI integration

Create a README, and attach the xdg-open files for right-click execution
and Processing in Unity.