Turn on ARC for the iOS target.
Remove non ARC support from OHTTPStubsResponse.
Remove non-ARC support from OHHTTPStubs.m
Better thread-safety for the requestHandlers array.

Now we won't read (enumerate) or write the array unless we're
synchronized on self.
Merge pull request #8 from github/arc-only

Remove non-ARC support and enable ARC for the iOS Target