Makefile updates: "make help" and more

- Add "make help" as the default makefile target.

- Add/rearrange a few makefile dependencies to avoid confusion.

- Split "resetdb" into separate "dropdb", "createdb", "migratedb"
  targets to allow easier automation of "createdb".

- Drop unused "update-submodules".  I don't know how they fit in here,
  and we'll probably do better if we never use submodules in the first
  place.

- Make "setup-git" purely optional, because local pre-commit hooks can
  sometimes be pretty obnoxious:

    $ git commit --amend
    sh: 1: node_modules/.bin/jshint: not found
Merge pull request #12 from dlitz/make-help

Makefile updates: "make help" and more