Don't add extra SSL constant if using iOS 7 SDK
Explicitly import SecureTransport.h.

For some reason SecureTransport.h wasn't included by Security.h, but it
looks like it's been available since at least iOS 5. So bring that in if
available.
Conditionalize explicit import of SecureTransport.h
Adjust SecureTransport import condition to be based on include that's already there
Merge pull request #4 from aleffert/master

Fix TCPConnection.m for iOS 7 by not redefining system constant errSSLClosedAbort