paths: Only set default pip-app dir if unset

Allow the user to have a different pip-app directory and only set the
default value if the environment variable `PIPAPP_DIR` is unset.
shell: `pip-app` works equally well with Bash

The shebang is not necessary when sourcing a file, only when executing
it.  `pip-app` works equally well with Bash and by skipping the shebang
it can be used on a machine where `zsh` is unavailable.
Merge pull request #1 from dholm/feature/user-defined-pip-app-dir

paths: Only set default pip-app dir if unset
Merge branch 'feature/bash-support' of https://github.com/dholm/pip-app

Conflicts:
	pip-app.sh
Fixed license link.
Use bash in generated executables.
command: Use commands instead of functions

Replaces the various pip-app functions with a single function,
`pip-app`, that accepts commands just like `pip`.  One benefit of this
is that the user only has to remember a single command, `pip-app` and
pip-app itself has a help text describing each available command.

This change also introduces a command for listing installed applications
and their binaries.