Add first three tests for XML trees
Yay! :)
* Changes to XML::XPath::Grammar:
  - Combine [Reverse||Forward]Axis into Axis token,
  - Combine [Reverse||Forward]AxisStep into AxisStep
  because they're separated for no reason.
  - capture optional '@' from AbbrevForwardStep
* XML::XPath:
  - Set context in constructor
  - Allow overriding context in evaluate
  - Correct Expr to expect an array from ExprSingle
  - Child and descendant axes
  - Child and descendant abbreviations