Match registered users sending emails to tracker and add them to "created by"

[fross123]

Kind of a random feature suggestion/request I guess, and relatively new to GitHub so please bear with me. I was able to add the above feature by trying to match the user with an email if "best_task" is none (So it only tries to find a match if a new task is being created). I'm using django-todo as a public-ish support management tool.

The goal for me was to make it so users could send an email to the tracked email address and if they were in the system, the sender would show up in the task as "created by". This would allow comments/replies to be sent to back to them in response. Its only helpful if users already have an account, but for me most users communicating with the tracking email will be registered.

I'm not sure if there was already a good reason this is not implemented, but I'd be happy to submit a pull request if you think it would be a good thing to add!

[shacker]

@fross123 This sounds like a very good idea. I'd be happy to take a look at your pull request. Thanks in advance!