add ssl shutdown, make SSL_ctx injectable etc.

hey Ian,

Great work addressing the thorny issue of supporting SSL/TLS in ZMQ !
I made a few changes and I am requesting a merge -
1. As you pointed out, it made sense to not have an SSL_ctx per instance.
2. I added the missing SSL_shutdown.
3. Separated the network IO from the TLSZmq buffer IO in the samples to make the distinction clear.
4. Few other minor changes.

Thanks,
-Vikas
PS: I also plan to look at adding exception handling and some unit tests soon.