Update loader to use the new parser interface

Fixes this error:

```
Traceback (most recent call last):
  File "./main.py", line 4, in <module>
    import test1
  File "/Library/Python/2.7/site-packages/literate/core/loader.py", line 39, in importer
    code = parser.python(source)
AttributeError: 'module' object has no attribute 'python'
```