Refactor: reduce scope of $tag

Now it no longer needs to be reset at the end of each parser iteration.
Refactor: eliminate unused variable
Refactor: delete commented-out code

It's available in the revision history for anyone who still wants it.
Refactor: delete needless semicolon after closing curly
Fix bug in parsing attribute selector after negated selector
Refactor: separate variable for each regex

Compared to the previous approach of using a hash, this has the advantage
that typos will be detected by `use strict`.
Add tests for :nth-child(2n) and :nth-child(2n+1)
Support :nth-child(odd) and :nth-child(even)
Avoid XPath 2 features when translating `[attr$=tail]`

The `ends-with()` function was introduced in XPath 2.0, so using it reduces
the portability of the generated XPath expressions.  In particular, both
libxml2 (used by XML::LibXML) and XML::XPath implement only XPath 1.0.
Prepare for next release
Merge branch 'master' of https://github.com/arc/HTML-Selector-XPath into arc-master

Conflicts:
	Changes
	lib/HTML/Selector/XPath.pm
Merge branch 'arc-master'
Bump version