build: Set up phplint and phpunit tests

Add a custom 'composer test' script to run PHPUnit tests for the
library.
Remove "minimum-stability": "dev" to have composer use the stable
versions of the dependencies (i.e. PHPUnit 4.4.x instead of some
random dev version).

Add phpunit.xml.dist to configure PHPUnit.

See README.md for updated instructions on how to run the tests.

Bug: 73541
Change-Id: I431850b34206f307fe294afbec981643fd46376c