fix(xpath): Adjust text methods to match lxml

[James-LG]

Adds an itertext() method to XpathItemTreeNode that behaves like lxml's itertext method.

Also adjusted XpathItemTreeNode::text() method such that it matches the behaviour of lxml's text() method.

Other fixes:

• Orders xpath results in document order
• Text nodes automatically unescape commonly escaped characters such as gt; -> >
  • Added unescape_characters() and escape_characters() functions to easily move between them

Fixes #26