add `mgit install [path]`

[capr]

linux & osx:
install $0 $path or /usr/local/bin/mgit
windows:
copy both mgit and mgit.cmd to syspath

[capr]

For linux, also find git's autocomplete script, find a place to source both the git and mgit autoconplete scripts and source them there.

[capr]

Also a --soft option which makes symlinks instead of copying files would be nice, so as to develop on mgit while installed and not have to reinstall all the time.