Clean up DrupalSubContextInterface.

[pfrenssen]

DrupalSubContextInterface could use a little love:

• The docblock for the constructor is not correct, it uses the wrong parameter type for $context.
• it states that the constructor returns a string but a constructor cannot return any value since it returns the object itself.
• We can also add some documentation to help people understand its purpose.

[pfrenssen]

I also renamed the $context variable to $drupal. This better conveys what it contains, and this variable name is also used in the FooFoo example implementation .