change detail timestamp to reflect gauge updates and allow refresh fr…

[ngottlieb]

this is a first pass at https://github.com/AmericanWhitewater/aw-ios/issues/280.

what I've done is:

• add refresh control to the reach detail view that reloads that reach specifically
• changed the timestamp in the reach detail view to reflect last_gauge_updated from the API instead of the app-wide "last updated"
• add a "pull to refresh" call to action when the last gauge reading is over an hour ago

I should be very clear that I have essentially no idea what I'm doing in Swift or iOS development. So please tell me to change things if I should!

[Gregliest]

Looks good to me, thanks for handling this!