Added history support, Added samples for history request

[Ellerbach]

I needed to use the history on top of forecast for a personal project. As the Library is just great, I've decided to add the history support.
Sample has been updated as well
Let me know if you have any question.

[dknoodle]

Ellerbach, I really appreciate the code contribution. There were some changes to how you had things structured and some duplicate class models. I went ahead and added it myself. I can't publish a NuGet update from where I am right now but I will get that in the next few days. The code changes have been pushed though.

[Ellerbach]

Thanks, I found few bugs and few Properties missing in the Observation class (some stations have specific Properties not in the documentation), so I'll refork based on your commit and I'll correct the bugs.

[dknoodle]

@Ellerbach - Do you have the model changes you made? Could you please send a pull request with the missing properties and bug fixes you indicated?

Thanks, Daniel

[hlngo]

What is the progress for this PR? I have a similar Python version that basically does the same thing with this PR and query for ONE past day at a time. The issue I have with WU is that they enforce number of requests/minute. I am thinking about putting a timer (sleep b/w each request) but I am not sure that's OK with them. Have you guys thought about this?