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?
I've done two tests
- on a solution containing 20000 files of 30 lines each and a total of 4 TODO comments. It worked acceptably (it only added about 1 second to the opening of the solution)
- on a solution containing 1000 files and a total of 4000 TODO comments. Parsing the files was done in much less than 1 second; adding the comments to the Task List took much longer than that (above 15 seconds)

I will work on improving the performance for even larges solutions