Misc. tweaks made while porting Chatty to use MYNetwork.
* Allow -[BLIPConnection sendRequest:] to re-send an already-sent or received request.
* Allow use of the basic -init method for BLIPConnection.
* Some new convenience factory methods.
* Broke dependencies on Security.framework out into new TCPEndpoint+Certs.m source file, so client apps aren't forced to link against Security.