LP 1463943: Encode output messages.

LP 1463943: Encode output messages.

This commit adds support for an encoding attribute that can be
added to accounts in the configuration file.  When set, SIPServer
will encode response messages in the specified encoding.  Valid
encodings are those listed in the Encode::Supported man page.

This feature is useful if you have a vendor who cannot support
the default UTF-8 output by SIPServer.

An example is provided in SIPConfig.xml.  The lpl-sc account has
been set to use the ascii encoding.

This particular code was mostly taken from a patch to Koha's
SIPServer by Adrien Saurat <adrien.saurat@biblibre.com>,
Christophe Croullebois <christophe.croullebois@biblibre.com>,
and others.

http://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=9865

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>