Rewrite HarParser to parse in constructor

Currently, the `HarParser` class requires the object be instantiated and parsed separately:

```python
raw_saml = HarParser(json_str).parse()
```

It would be nice to change this so that parsing happens in the constructor and a function is called to get the SAML data.

```python
raw_saml = HarParser(json_str).get_raw_saml_response()
```

This will make it more consistent with the other classes.