Fix to allow plugins to extension JSON schemas when loaded.

The implementation there was using a relative directory without being
explicit about what it was relative to.  This worked for plugins that
are bundled with ArchivesSpace, but not for plugins that were added to
the distribution after the fact.
Modify the plugin_helper to handle zero_to_one record types.

Currently it assumes that plugin-added record types will always be
arrays.  Allow it to be a single value (or nil) too.
fixes to ead importer
Merge branch 'master' of https://github.com/archivesspace/archivesspace
Merge pull request #64 from hudmol/plugin-schema-extension-fix

Plugin schema extension fix
Merge pull request #65 from hudmol/plugin-tweaks

Plugin tweaks - bug fixes and running tests
Merge pull request #66 from hudmol/plugin-helper-single-record-fix

Modify the plugin_helper to handle zero_to_one record types.
fixing utf-8 on EAD physdesc import
Merge branch 'master' of https://github.com/archivesspace/archivesspace
fixing DAO import on EAD and adding DAOGRP and DAODESC support. Also more fixes for empty/self closing tags
have loading of nested_records repect the model association's order