feat(AstParser): Made the AST parser private to the scope

closes #648
feat(scope): watch watches expressions (String) only
feat(directives): remove refs to AstParser, watch strings
feat(interpolate): the service now returns an expression only
refactor(ngPluralize): minor tweaks and code style
feat(ngPluralize): support the new interpolation service

This commit also:
- remove the previously registered watch,
- add an expression cache