Refactor
Add basic location matching to test cases
Refactor
Clean up tags
Refactor
Fairly major re-jigging of the AST interface.

Highlights:

- comments and tags are now just held in arrays on the nodes
- Ast::Comment now represents a single line rather than the whole block
- match_location? method added to Test::Case and AST nodes so we can
  filter on location

We really need to tidy up the AST node attributes soon.