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Changes for release 2.0.0, to support ZMQ/4.x and security

* Updated provisional version number to 2.0.0 since this is a major
  expansion of the CZMQ API. The changes are mainly tools to support
  encryption via the libzmq CURVE mechanism, but also some new API
  constructs to make it simpler to do common messaging work.

Bug fixes and cleanups of codebase:

* zconfig_load () failed if the ZPL file contained a blank line.

* zconfig_locate () allows and ignores initial '/' on pathname.

* Added zsys_vprintf () to centralize all variable string formatting;
  this code was duplicated many times across the project.

* Updated zmsg interface documentation to be more detailed.

* Removed deprecated zstr_sendfm () method.

* Removed deprecated zstr_sendf () method.

* Removed definition of ZMQ_IGNERR and added zloop_set_tolerant ()
  to configure a specific poller as error-tolerant.

* Removed deprecated zframe and zmsg zero-copy methods; this API was
  too complex and split over several classes, and IMO if we want this
  optimization it should be in a new zero-copy class with a clean API.

* Removed deprecated zsocket zero-copy methods, for above reason.

* Removed deprecated zlist_copy method.

* Renamed zframe_print_to_stream to zframe_fprint and kept old name
  as deprecated macro.

* Removed (not deprecated) zctx_set_hwm () as its semantics were not
  consistent, and confusing to people.

* Removed (not deprecated) zctx_hwm (). We now have separate HWMs for
  normal socket SND and RCV, and for inter-thread pipes. There is no
  need to provide accessor methods for these.

New API methods to simplify common messaging tasks:

* Added zpoller class to provide new minimalist interface to zmq_poll.
  This class makes it very simple to read off a set of sockets, which
  is an 80% usecase.

* Added zstr_sendx () to send multiple strings in one call. Inspired
  by PyZMQ's multipart send method.

* Added zstr_recvx () to receive multiple strings in one call. We do
  a lot of multipart string reads; this makes it simpler for simple
  cases.

* Added zclock_timestr () to provide date/time as printable string.

* Added zmsg_append () that nullifies caller's frame reference; this
  is more consistent with the API style than zmsg_add (), which is
  deprecated.

New classes used for security infrastructure:

* Added zchunk class (from FileMQ), for managing memory blocks.
  This is a simpler model than zframes, which are optimized for
  message data.

* Added zhash_comment () to add comments to a serialized zhash file.

* Added zhash_refresh () to reload a backing file if modified on disk.

* Added zconfig_comment () to add comments to saved config file, and
  added code to save such comments from file when loading it.

* Moved zfile (char *filename) methods to zsys, to make place for
  a new zfile class based on the CLASS object model. The old methods
  are still provided but are deprecated.

* Added zfile class (from FileMQ), for managing files. This provides
  a rich file class with metadata and operations. This is a component
  for directory management (zdir).

* Added zdir class (from FileMQ), for managing directories. This lets
  us work with filesystem directories. I wanted this to be able to
  load certificates for security authentication.

* The zdir and zfile classes are slightly stripped down from their
  FileMQ originals; no symbolic links, no directory difference tools.

* Unfortunately to keep the old zfile_size and zfile_time methods, I
  had to use zfile_cursize and zfile_modified in the new class. This
  is a little clumsy.

* Added zsys_file_modified () to return timestamp for a filename.

* Added zcert class to work with Curve certificates. A certificate is
  a public + secret key pair, plus metadata like name, email, etc. and
  is saved as two files, a public text file and a secret text file.
  Certificates are stored in ZPL (ZMQ RFC 4) configuration file format.

* Added zauth class to automate ZAP authenticators. This class does a
  simple plug and play authentication of clients against certificates
  stored in the $HOME/.curve directory. This can be extended over time
  with other designs.

* Extended zsockopt generation to allow use of binary keys in CURVE
  key options (e.g. zsocket_set_curve_secretkey_bin ()).

* Added addons/makecert.c tool to generate CURVE certificates.