Make CDATA more accessible

[paulvt]

CDATA is usually just used as a way to escape text, but there is no real nice API to reach it in Ox. For example, compare <foo>bar</foo> and <quux><![CDATA[<garply>nice</garply>]]></quux> when trying to reach the text/literal data:

foo_xml.text
#=> "bar"
quux_xml.nodes.first.value
#=> "<garply>nice</garply>"

I am aware that <quux> could have had multiple CDATA nodes, but the same holds for <foo> containing mixed strings and elements, then #text also returns the first string node. Additionally, I should also have checked that quux_xml.nodes.first is even a CData node.

[ohler55]

Reference issue 41 and closing this one.