add ability/instructions for installing marionette extension per version

This is part of Bug 1016537 (https://bugzilla.mozilla.org/show_bug.cgi?id=1016537). I've updated the marionette_extension python package to deal with v1.4, and now we need to add the ability to install the right marionette version according to firefoxOS version.

This patch gives instructions on how to install the extension, and it is safe to run this modified script if you already have marionette installed. It will just say it's already installed and won't try to reinstall it.

The script requires the user to input the version number, since there isn't a known way for us to programmatically figure that out with just adb.