Tidying up the code in the issue hook
Adding option to disable interactive assignment on New Issue form
Don't fall back to global default if the project specifies no default assignee

Previously, the behaviour was to use the assignee specified in the configuration
screen if the project had no default assignee set. I don't think this makes
sense; you may want to be able to set a blank assignee in certain projects, and
it's unlikely you'll have so many projects that setting an assignee on each one
will be that difficult.
Bumping to version 0.3.0 to reflect change in behaviour