Updated the PushResponse object to use DateTime objects instead of strings. The client now takes an optional param of a TimeZoneInfo object. If it's null all times returned will be UTC.

Signed-off-by: Adam Yeager <adam.yeager@gmail.com>
Added methods for getting the user's subscriptions, unsubscribing, and getting channel info.

Signed-off-by: Adam Yeager <adam.yeager@gmail.com>