Fix pre-commit workflow.

[Drew-Kimberly]

This PR:

1. Adds php_codesniffer as a dev dependency.
2. Updates run_phpcs.sh so that it will properly run PSR2 validation against the codebase.
3. Updates run_phpunit.sh so that it will run the Unit test suite.
4. Updates some documentation in README.md.

I'd like to include a pre-commit Git hook so that the pre_commit.sh script is automatically fired, but thought I'd ask first. Is this something you'd like included?

[jmglsn]

Hi @Drew-Kimberly,
thank you for your PR, actually we removed the codesniffer dependency in #123 and maybe we're also going to remove the pre-commit hooks, actually the checks should be configured at repository level instead.
Sorry, so there is no need to merge this PR.