Update tests to use latest version of php-markdown

I copied the source code from php-markdown (just the Michelf directory) and put it in mdtest/Implementations. The existing files were modified to be wrappers so that the test harness would work with (minimal) modifications.

The tests don't do very well unless you check the "Normalize" option. Is that normal?

It may be a good idea to convert the php-markdown source into a submodule in the future, so that it's easier to pull the latest changes.