Simplify test for callable
Simplify test for callable

- Closures and other objects implementing __invoke are callable
- Function method_exists accepts any 'object' parameter
- Strong typing is coming in php7 for objects
silexphp/Pimple,jpauli/Pimple,jpauli/Pimple,gomboc/Pimple,jpauli/Pimple,mnapoli/simplex,arduanov/Pimple,gomboc/Pimple,michabbb-backup/Pimple,arduanov/Pimple