Add's the configuration file option "dump_json" or command line
options [-j|--dump-json] to have swift-dispersion-report output
the report in json format. This allows the dispersion report to
be more easily consumed elsewhere.
There's also a few pep8 fixes and removal of unused imports.
Change-Id: I2374311ccbef43e6bbae24665c9584e60f3da173