Escape value of string metric according to XML specification

When the value of a string metric contains characters ('"', '<', '>', '&', etc.),
that must not be present in XML attribute value, convert that characters
to special entities ('&quot;', '&lt;', '&gt;', '&amp;', etc...)
in the gmond status output
Merge pull request #131 from miihael/master

Escape value of string metric according to XML specification