fix($compile): allow access to the controllers of transcluded elements from child elements.

Additional API (backwards compatible)
- Injects `$transclude` (see directive controllers) as 5th argument to directive link functions.
- `$transclude` takes an optional scope as first parameter that overrides the
  bound scope.

Deprecations:
- `transclude` parameter of directive compile functions (use the new parameter for link functions instead).

Refactorings:
- Don't use comment node to temporarily store controllers
- `ngIf`, `ngRepeat`, ... now all use `$transclude`

Closes #4935.