Report section proof of concept

Created a proof of concept on how Report section on the dashboard should
work.

There is still a couple of big question marks on how right this approach
is to the problem. There is some cross-logic on Generator and Formatters
going on which will have to be tackled in the future in order a user to
be able to override the Generator.run() method.

One good idea could be to create a CeleryGenerator class which would be
capable to dispatch a report generator to a different machine.

Examples on how to create reports can be found on
oscar/apps/catalogue/reports.py.

There is also an autodiscover function which will pick all the available
report generators and make them available for a user to run on the
dashboard reports page.