Enhanced the handling of UTF-16, UTF-16LE and UTF-16BE encodings. Now

* encoding.c, include/libxml/encoding.h: Enhanced the handling of UTF-16,
  UTF-16LE and UTF-16BE encodings.  Now UTF-16 output is handled internally
  by default, with proper BOM and UTF-16LE encoding.  Native UTF-16LE and
  UTF-16BE encoding will not generate a BOM on output, and will be
  automatically recognized on input.
* test/utf16lebom.xml, test/utf16bebom.xml, result/utf16?ebom*: added
  regression tests for above.
diff --git a/result/utf16lebom.xml.sax b/result/utf16lebom.xml.sax
new file mode 100644
index 0000000..e699631
--- /dev/null
+++ b/result/utf16lebom.xml.sax
@@ -0,0 +1,8 @@
+SAX.setDocumentLocator()
+SAX.startDocument()
+SAX.comment( This file is encoded in UTF-16LE )
+SAX.startElement(repository, repositroy_id='test')
+SAX.startElement(namespace, name='test')
+SAX.endElement(namespace)
+SAX.endElement(repository)
+SAX.endDocument()