Fixes bug 1322255 - Sticky headers render over inspector

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.