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?
Since the hunt for task list comments should begin as soon as a solution is loaded I had to basically subscribe to the solution opened event during the package's initialization. I did not find any way to obtain an instance of ProjectFactory in that phase (and no way of getting VSLanguageService either, otherwise I could have manually instantiated the proj factory).

If you know of a way to get a ProjectFactory object in that phase I'd greatly appreciate the help.

P.S. It took me so long to answer this questions because I wanted to check again, to make sure hadn't missed something the first time I tried.