Update OpenHMIS project diagram and API overview.

* doc/openhmis.dia: Update diagram to reflect current status of
  project, and to use new categories and terminology from [1].
  This is the version posted in [2] (as the PNG below).

* doc/openhmis.png: Version generated file for convenience.

* API.md: Update to reflect Tony Zhang's recent commits adding the
  HUD 2014 schema diagram (see doc/2014StandardOpenHMIS.*).

* doc/schema-diagram.jpg: Rename to...

* doc/current-schema-diagram.jpg: ...here.

References:

    [1]
        From: Karl Fogel
        To: Dave Totten
        Cc: OpenHMIS Discuss
        Subject: Re: Defining what is and is not "OpenHMIS".
        Date: Tue, 18 Nov 2014 12:15:10 -0600
        Message-ID: <87ppck2x5d.fsf@floss.red-bean.com>

        http://openhmis.pcni.org/developers/discussion-group?place=msg/openhmis-discuss/SxBFDo-55ss/6BQGN0nEQkMJ
        Same as:
        https://groups.google.com/a/openhmis.org/d/msg/openhmis-discuss/SxBFDo-55ss/6BQGN0nEQkMJ

    [2]
        From: Karl Fogel
        To: OpenHMIS Discuss
        Subject: Updated diagram, with new categories and current statuses.
        Date: Thu, 20 Nov 2014 23:07:59 -0600
        Message-ID: <87tx1tta34.fsf@ktab.red-bean.com>

        http://openhmis.pcni.org/developers/discussion-group?place=msg/openhmis-discuss/9Ai4lT427TY/OU6RBB96I3gJ
        Same as:
        https://groups.google.com/a/openhmis.org/d/msg/openhmis-discuss/9Ai4lT427TY/OU6RBB96I3gJ