Make gerrit.py executable

Add executable bit and use env to figure out where python may be
installed.
Possibly define project without an option

It would be cool to not have to pass the --project option. Use a
positional argument and in case --project is not passed use that as the
project. This keeps backwards compatibility but adds some confusion.
Also display the id in change listing

Id numbers for gerrit changes feel useful so add it to the listing. In
the same commit, have the same spacing used for the count of changes and
have the id padded to 7 spaces to keep a nice UX
Minimal unix support

Add some linux support through simple if then else clauses. This is more
of a PoC and it only support the GNOME desktop environment on linux. A
more scalable approach should be used if better OS support is required