Move Pingdom requests back into Pingdom client class

This effectively reverses 3ae441 (from the
alphagov/backdrop-pingdom-collector) and changes the tests to just mock
the request method in the class. As far as I can see it was moved out of
the class to make testing easier but this can just be done in a slightly
different way.
Merge pull request #60 from alphagov/move-requests-in-pingdom

Move Pingdom requests back into client class
Fix namespacing for PyPi installs

See https://github.com/alphagov/performanceplatform-client/pull/5
Merge pull request #61 from alphagov/fix-namespacing

Fix namespacing for PyPi installs
Use performanceplatform client PyPi library

- Change import path for DataSet and JsonEncoder
- Remove old tests
- update requirements.txt
Add instructions on development setup

See https://github.com/pypa/pip/issues/3
Merge pull request #62 from alphagov/use-standalone-client

Use standalone performanceplatform-client from PyPi