Segment exploring capability

I came across your excellent Strava V3 library for a pet project and as luck would have it the segment explorer API I was after was one of the few that wasn't supported.

I added a new API call on FRDStravaClient+Segment : fetchSegmentsWithRegion:activityType:success:failure
I also added a basic explorer into the sample app.

A couple of things to note:
I used a MKCoordinateRegion to specify the bouncing box for the desired segments. This does introduce a dependancy on MapKit but I couldn't think of any better data structure which wouldn't cause confusion?

I had trouble with the decodePolyline method on StravaMap which would crash due to an out of bounds issue.
I introduced a broadly duplicate method that returns an MKPolyline which appears to not have the same issue.