Specs for XML::Element#text.
Added XML::Element#inner_text.

This method can be used to retrieve the text of the given node only. In other
words, unlike Element#text it does not contain the text of any child nodes.
Added Node#previous_element and Node#next_element.

These methods can be used similar to #previous and #next expect that they only
return Element instances opposed to all Node instances.
Added Node#remove.

This method can be used to remove individual nodes without first having to
retrieve the NodeSet they are stored in.