Add timezone parsing / serializing

[jorroll]

This PR updates the full_tz branch with correct timezone parsing / serializing. I'll add some comments in the code to explain some of the changes. It uses the responds_to?(:time_zone) method you already had in place to check for ActiveSupport.

• With this update, the full_tz branch might (?) have full time zone support (Supports parsing, serializing, and iterating time with zones--I think that's everything).
• Without this update, I think the full_tz branch only supports iterating time with zone's, which I believe the master branch already supports (?).

[jorroll]

Hey @pacso, it's quite possible that the comment I left on Jan 14th, 2019 was the last time I seriously looked at Ruby code. I'm definitely rusty at this point and not in a position to work on this merge request. Feel free to either close this or pick up where I left off (feel free to make use of the code here however you want). I switched to typescript and now maintain rSchedule.

[suzumejakku]

Hi, It would be very nice to have this official support of timezone parsing; can the author init a merge ?

Edit: I see that #526 should take care of it and is about to be merged. Fingers crossed :)