Move Adapter\Stub to Source\ArrayObject.

Little change of plans here:

- An Source\AbstractSource weas made implementing a Chain of
  Respposability for other Sources.
- A source now is only able to **read** values as the change of name
  already implies. This should make the API more stable, easy to
  implement and test.
- Removes unused Exception\MissingBehavior.
Move Adapter\PHP to Source\GetEnv.

- Removes setting values functionality.
- Ensures null is returned in case the environment variable does not
  exist.
- Changes name to one that describes better that that source of
  information really is.
Move Adapter\Apc to Source\Apc.

At least I have not change this adapter/source name. :)

- Add "lstrojny/phpunit-function-mocker" to mock PHP functions.
- Remove write/delete functionality.
Removes Adapter\Rules, Behat tests and unused Exceptions.

Cause without write/delete functionality, rules, behaviours (along with
the Mediator crap) were just stupid.

Removes "Respect\Validation" and "Behat" from composer.json also.
Updates README to match current (desired) status.
Merge branch 'remove-write-ability' into develop