Make svn_wc_parse_externals_description3 available from Perl bindings.

* subversion/bindings/swig/include/svn_containers.swg:
  Add output typemap for APR array of svn_wc_external_item2_t.

* subversion/bindings/swig/perl/native/Wc.pm:
  Document function svn_wc_parse_externals_description3.
  Add the magic to access struct svn_wc_external_item2_t
  as an object and document its methods.

* subversion/bindings/swig/perl/native/Core.pm:
  Fix a typo that prevented the use of 
  _p_svn_opt_revision_value_t objects.

Patch by: Roderich Schupp <roderich.schupp{_AT_}gmail.com>


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1443783 13f79535-47bb-0310-9956-ffa450edef68