use lein-sub instead of shell scripts clean, install, package, push test

The motivation is to move closer to orthodox leiningen-based workflow. Most of the shell scripts in the script/ subdir add complexity but not value, since a standard leiningen plugin, lein-sub, is available that does what they do.  This change adds and configs lein-sub and removes clean, install, package, push, and test files from the script/ directory.  Also adds doc/developers.md showing how to use lein-sub.

-Gregg