Option to specify a custom XSLT to be used for pretty formatting

This backward-compatible change allows specifying a custom XSLT. It also adds a Gemfile and updates the Rakefile to be compatible with current versions of Rake so that the tests run. The Readme has been updated to document the feature.

Essentially, it adds an :xslt option:

    doc = Nokogiri::XML('<foo><bar>baz</bar><bar>buzz</bar></foo>')
    doc.human(:xslt => File.read("some_other.xslt"))

All new and existing tests pass.