Added function get_class_methods

I run the following tests:

```bash
$ PYTHONPATH=~/test_hippy/pypy py.test -r s ./test_phpt/Zend/bug32296.phpt ./test_phpt/Zend/bug38942.phpt ./test_phpt/Zend/bug43483.phpt ./test_phpt/classes/autoload_002.phpt test_phpt/ext/standard/class_object/get_class_methods_* test_phpt/Zend/get_class_methods_00* test_phpt/ext/standard/class_object/method_exists_*
```
Here I have a doubt as in some other methods like `_lookup_method` the method is searched first in the `contextclass`. But the current implementation passes all tests.