Translate XML entities in iFCs, and handle CDATA nodes properly.

We recently tried to just treat XML values as just text strings,
and not decode any &gt; or similar entities in them, on the basis
that we don't encode them when inserting iFCs. But this means we
have no way to express a literal "<", and so can't match SIP
headers with a value of e.g. "<sip:example.com>". This commit reverts us to the old position.
Code review feedback
Merge pull request #108 from Metaswitch/xml_cdata

{Reviewer: Matt} Translate XML entities in iFCs, and handle CDATA nodes properly.