Added the basics for a PID controller, with no I implemented yet, as initially we are only using P and D.
Added basic unit-testing to the PID controller.
Added test for the case of array input / output.
Fixed the derivative part, after the unit-tests failed.
Added more tests, and modified existent tests to test I behaviour.
Implemented "I" behaviour. Completing the PID controller.
Fixed no new line at end of file. (Hope so at least)
Fixed no new line at end of file. (Hope so at least)
Merge branch 'issue18' of github.com:faitdivers/pyao into issue18
Merge pull request #27 from faitdivers/issue18

Closes issue 18. Merging based on aproval from @JaccovdS