Improve namespace support for XMLReader

Methods added:
* XMLReader::move_to_attribute_no(index): move internal cursor on the attribute
specified by its position
* XMLReader::move_to_attribute_ns(localName, namespaceURI): move internal
cursor on the attribute specified by its name and namespace
* XMLReader::get_attribute_no(index): get the value of the attribute specified
by its position
* XMLReader::get_attribute(name): get the value of the attribute specified by
its name
* XMLReader::get_attribute_ns(localName, namespaceURI): get the value of the
attribute specified by its name and namespace

Methods affected:
* XMLReader::move_to_attribute(name): position (name as Fixnum) is no longer
supported (use move_to_attribute_no instead)

Note:
* XMLReader::[] is not modified (index or name can still be used)
For compatibility, Reader::move_to_attribute still accepts a Fixnum
argument but emits a depreciation warning which invites to use
Reader::move_to_attribute_no.
Merge pull request #51 from julp/reader_ns_support

Improve namespace support for XMLReader