Add a --debug-level option

Adding an argument to --debug was a bad idea, because it's ambiguous and could break existing commands:
`opam install --debug xxx` would take `xxx` as the debug argument (and fail).
Improved package listing commands yet again
Merge pull request #1513 from OCamlPro/more-cl-options

Improved package listing commands yet again, and fixed --debug
Merge pull request #1512 from rdicosmo/update-documentation

Update documentation
Better self-package def description

I use this a lot to test the new pinnings
Fixed Makefile for running outside of git

Also, not refreshing OpamGitVersion unless you 'make clean', it may get a bit
out of date but, otherwise, we'd be rebuilding everything on every commit.
Or we could make only OpamArg depend on it rather than the base lib...
Fixed 'opam pin add .'
Merge pull request #1514 from OCamlPro/pin-fix

small pin & build fixes
Updated future default values for external solver preferences.