Pretty print XML

[jargonjustin]

Adds an XML processor to pretty print XML response types.

It looks like there was some previous discussion about this in #34. While this change uses the new plugable processor API, it doesn't address:

However, the method used here inserts a lot of blank lines. A bit less agressive formatting is needed.

A little research on pretty printing XML with Python revealed a few options:

1. Use xml.dom.minidom which apparently does a mediocre job in many cases
2. Use PyXML or lxml, either of which would require adding a dependency
3. Write my own pretty printer, which entails a decent amount of complexity.

I've opted to go with the first option above as a starting point. In my testing, it has handled machine-generated XML with no extraneous whitespace well. Please let me know if another option would be more appropriate.

[jkbrzt]

Thanks, @jargonjustin. This is a good starting point.

[jkbrzt]

I've just added some unicode handling and it seems to work very well. Thanks and sorry for the long wait!

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