Xero reports - wrapper class to do the ugly parsing for us

[timbledum]

I have added:

• xero/report_wrapper.py, which contains the class to wrap and clean up the returned value of the reports calls
• examples/xero_report_sample.ipynb, an iPython notebook which demonstrates the wrapper in action
• a minor tweak to init.py to enable the wrapper to be imported easily.

I am not familiar with creating tests, so if tests are required any guidance here would be greatly appreciated. Also happy to add documentation to the readme and happy for things to be moved around/renamed.

Cheers Marcus