Implemented quoted_printable_decode and quoted_printable_encode

Tests run:

```bash
$ PYTHONPATH=~/test_hippy/pypy py.test -r s test_phpt/ext/standard/general_functions/002.phpt test_phpt/ext/standard/general_functions/006.phpt test_phpt/ext/standard/strings/bug62462.phpt test_phpt/ext/standard/strings/bug64879.phpt test_phpt/ext/standard/strings/quoted_printable_*.phpt testing/test_string_funcs.py
```