Add a factory function to generate the proper paerser object

As an alternative to doing this

```
from tree_hugger.core import PythonParser

pp = PythonParser()
```

We also suggest having something like this

```
from tree_hugger.language_factory import get_parser

pp = get_parser('Python')
```

As most of the parsers expose a very similar API so having a factory makes sense in this regard. 