Added checks to make sure that we are not swizzling the NSURLConnections more than once.

This would happen if say you had multiple instances of ApigeeClient created.  In such a case where this would happen, the methods and the override methods end up pointing to the same method implemenation and you see a broken recursive cycle.
Merge pull request #81 from RobertWalsh/master

Added checks to make sure that we are not swizzling the NSURLConnections...