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?
Have you tested the performance on large solutions? I'm afraid it can take long time to process hundreds of thousand of lines.