add the ability to look at assigned tasks

[theonesean]

Lots of team-oriented providers (including all of the ones we currently support) allow the ability to assign tasks to a specific person. This would be helpful data to have and allow for a number of uses:

• assign/unassign this task to me
• assign/unassign this task to someone else
• only display the tasks assigned to me
• possibly display tasks assigned to me in a different color/emphasis

This would involve updating the Task object and the provider framework. For providers that have an easy "is this task assigned to me" API call, implementation will be simple. If not, we'll have to abstract over a number of API calls and munging for this to work. But I think it'll be useful.