Charmbrowser view now cancels inflight store requests when it's destroyed
Add tests for the new transaction id's being returned and for the cancel in flight request method added to the charmworld api
Merge pull request #348 from hatched/sticky-headers-1322255

https://bugs.launchpad.net/juju-gui/+bug/1322255

This adds a method to abort in flight requests from the charmworld api. So now when the charmbrowser view is destroyed it can abort any in flight requests so that it doesn't render after it's been destroyed.

#### To QA
Follow the steps in the bug to try and reproduce.
Everything else should continue to work as expected.

#### Note
Charmworld responded too fast for me to properly test this bug so I needed to throttle my network connection down manually.