Remove "listallexecutablesinpath.swift" and turn it into a Gist. List it as an example in the README.

It was way too wordy to use as a test.
Include assert.sh from http://github.com/lehmannro/assert.sh for shell unit testing.

Also included the required licenses.
Add shell unit tests and update test scripts.

Run SwiftShellTests/Scripts/runtests.bash to perform all unit tests.