Allow initial Filter Criteria to specify SIP header matches as triggers to send a message to an application server.

Function is as defined in TS 29.228, including the SIPHeader, Header and Content nodes.

Edge cases with newlines and commas not yet resolved.
All UTs passing
Running astyle --style=ansi -s2 -G -k1 -j as per the style guide
Code coverage for error cases
Applying code review feedback
Merge pull request #101 from Metaswitch/sip_header_ifc

{Reviewer: Mike} Allow initial Filter Criteria to specify SIP header matches
Fix valgrind warning about uninitialised variable
Merge pull request #102 from Metaswitch/sip_header_ifc

[No reviewer required] Fix valgrind warning about uninitialised variable