irssinotifier.pl: move sending to its own thread

Even with the wget timeouts irssi will still block for a while
during network outages. Move sending logic inside its own thread
to avoid this.
irssinotifier.pl: use perl for sending instead of wget

Running shell programs with unvalidated outside input is scary,
use LWP::UserAgent instead for sending.
irssinotifier.pl: use perl for encrypting insteal of OpenSSL

Running shell programs with unvalidated outside input is scary,
use Crypt::CBC instead for encrypting.