Add methods to set the Auth-Application-Id or Acct-Application-Id

Graeme,

Please can you review this fix to add methods to set the Auth-Application-Id and Acct-Application-Id.  This is needed to fix https://github.com/Metaswitch/homestead/issues/74.  As part of this, I've also

*   exposed the freeDiameter vendor, application, command and AVP data structures
*   moved the iterator to use these
*   added `get_str_from_avp` and `get_i32_from_avp` methods to the AVP class (to make UT easier) - these were already present on the Message class, and I think will be useful.

UT-ed through the homestead change (to follow shortly).

Matt