Problem: need way to customize client API

There are some methods which are too painful to generate, as we
don't yet have clean abstractions for them.

Solution: allow user to add custom code to generated API, using
<custom> tag in client model. Syntax:

    <custom filename = "somefile.xml" language = "C" />

where somefile.xml contains <header> and <source> sections included
into API header and API source engine respectively.
Merge pull request #182 from hintjens/master

Problem: need way to customize client API
Problem: crashed when trying to delete expired tickets

Solution: when ticket expires, there's no need to delete it, as
this is already done by zloop. Thus, clear our own ticket handle
when ticket expires.
Problem: client hands in destructor, on interrupt

As client destructor does a handshake with its background thread,
to allow a clean disconnect when possible, it hangs if the user
interrupted the process with Ctrl-C (as it never gets a reply
signal in s_accept_reply ()).

Solution: if zsys_interrupted is true, do not wait for a reply,
just return -1 immediately. The destructor then completes happily.
Merge pull request #183 from hintjens/master

Problem: crashed when trying to delete expired tickets