Allow ad hoc reports to bypass XML parsing.

Attempts to address #99
The generate method can now be passed with a "raw" flag to indicate that
the response should not be parsed as XML for exception handing.
Example usage:
```
  config = AdhocReportConfig.new('audit-report', 'raw-xml-v2', site.id)
  report = config.generate(nsc, 3600, true)
```