Task List comments

After seeing how desired this feature is at [user voice](http://vfpt.uservoice.com/forums/247560-general/suggestions/5773087-todo-comments) I decided to attempt to implement it.

This is what I have so far. I'd be willing to invest some more time into improving it by implementing:
- updating Task List when files are modified from outside of VS
- navigate from task list entry to location in code file
- other useful ideas

Those are just things to experiment with. I don't know how hard/possible it would be to implement them.

I don't have the means to test this on VS14, so I'd appreciate if you could do that. 2012 and 2013 work fine.

Thanks!

P.S. Just let me know if I should change anything

**EDIT**: I noticed AppVeyor failed to build this PR for the same reason it failed to build others 3 days ago (something about "NUnit test failed"). Is there something I can do to fix it or is it normal?
Unfortunately it might not be possible. This class is needed during the initialization of the package so that we can start looking for task list comments as soon as the solution is opened. But it makes use of the TaskProvider class, which behaves a bit strangely. From what I've seen only the first instance of TaskProvider that is created actually has any effect on the Task List. If we add tasks to instances created after the first, they don't show up in the Task List. I have not found an explanation for this yet.

I have very little experience with how MEF is used by VS, so if it is possible to get an instance of this class (TaskListManager) in the package initialization phase AND use the same instance in TaskListCommentFilterProvider, I'd be very grateful for your advice.