…d orca

To help plotly.py discover orca, we need something on the PATH named `orca`,
not `orca-X.Y.Z...`.

Also add references about the Linux system PATH and AppImage

…anently

Prior instructions for adding orca to the PATH were not persistent between sessions.

The PATH manipulation instructions are delegated to a nice article on `computerhope.com`

Updated the Mac OS PATH installation logic to be a bit smarter:
 - Don't make any changes if `orca` is already on the `PATH`
 - Only copy `orca.sh` to `/usr/local/bin` if orca is installed as an application (not if it's installed through conda or npm)
 - Perform `orca.sh` copy with administrator privileges to avoid permission denied errors. This is done using Apple Script, which launches a nice GUI password prompt.

Updated README instructions to reflect the new installation process

This requires extra system libraries and xvfb.

Instructions now match our deployment Dockerfile configuration