Delay refresh when task add/edit dialog visible

Modifies auto-refresh to postpone the call to `window.location.reload()`
if the DOM contains a div matching `div.workitem-dialog`. In this case,
we re-test every 5 seconds to see if the dialog has closed.

***

I think the extension should CTFO if the user is potentially
adding/updating a task.
Merge pull request #2 from matt-holden/safe-refresh

Delay refresh when task add/edit dialog visible
Merge pull request #1 from matt-holden/master

Restrict extension execution to TFS task board
merged in changes sent by Christion Richert
added contributing instructions to README
latest build
README
README
added changelog and contributors to readme
added check for upper and lowercase 'taskboard' in manifest
latest build with manifest fix