Added non-blocking TLS-connections support.

The `circe-tls-make-stream' function tries to mimic the
`make-network-process' function in it's parameters, but to behave
closely to standard `open-tls-stream'.

Warning: tls-success is not correct by default in Emacs 24.3.1 with
openssl 1.0.1e or 1.0.1f, and it's important for this new function,
so it should be changed to "- Handshake was completed\|Verify return
code".
circe-tls-try-single-command: Also add a process sentinel.

Emacs does not guarantee a process filter to run if the process
fails without emitting any output. Use a sentinel to check for
connection failures.
Fixed some of circe-tls issues

Added a check for process attached to a target buffer, so now it must not
assign the same buffer to different processes, even if they are running
simultaneously.

And removed a call to fail-thunk from the temporary process filter, so now
only it's sentinel calls the fail-thunk (and it does not open excessive
connections on error).