Make lua scripts installable without git

[ChristianBirzer]

Currently installing the lua extensions requires git to be installed. This is rather common on developers machines but very uncommon on "normal end users" machines.
I suggest to add a fallback mechanism to the script manager that allows downloading the scripts as zip files from github and unpack that. I guess, unpacking zip should be available on all systems out of the box.

If I get positive feedback on that issue, I'd try to implement it.

The first release of that feature should be bundled with a dt release...

What do you think...?

[wpferguson]

See #352