Fixes ticket:68.
  NOTE that this involves an API change!  Entity declarations in the doctype
  now generate events that carry two, not one, arguments.

Implements ticket:15, using gwrite's suggestion.  This allows Element to be
subclassed.

Two unrelated changes, because subversion is retarded and doesn't do
block-level commits:

  1) Fixed a typo bug in previous change for ticket:15
  2) Fixed namespaces handling in XPath and element.  

    ***** Note that this is an API change!!! *****

    Element.namespaces() now returns a hash of namespace mappings which are
    relevant for that node.

Fixes a bug in multiple decodings

The changeset 1230:1231 was bad.  The default behavior is *not* to use the
native REXML encodings by default, but rather to use ICONV by default.  I know
that this will piss some people off, but defaulting to the pure Ruby version
isn't the correct solution, and it breaks other encodings, so I've reverted it.

* Fixes ticket:61 (xpath_parser)
* Fixes ticket:63 (UTF-16; UNILE decoding was bad)
* Cleans up some tests, removing opportunities for test corruption
* Improves parsing error messages a little
* Adds the ability to override the encoding detection in Source construction
* Fixes an edge case in Functions::string, where document nodes weren't 
  correctly converted
* Fixes Functions::string() for Element and Document nodes
* Fixes some problems in entity handling

Addresses ticket:66

Fixes ticket:71

Addresses ticket:78
  NOTE: that this also fixes what is technically another bug in REXML.  REXML's
  XPath parser used to allow exponential notation in numbers.  The XPath spec
  is specific about what a number is, and scientific notation is not included.
  Therefore, this has been fixed.

Cross-ported a fix for ticket:88 from CVS.

Fixes ticket:80

Documentation cleanup.  Ticket:84

Applied Kou's fix for an un-trac'ed bug.

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git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@11548 b2dd03c8-39d4-4d8f-98ff-823fe69b080e