Update Rakefile for newer rake version

The check_dependencies task no longer exists.
rake/rdoctask was removed and is not rdoc/task.
Add Gemfile
Bundle nokogiri 1.6.x to avoid segfault when running tests

Nokogiri 1.5.x uses the old libxslt (as opposed to libxslt2) library which is buggy.
Running tests that execute the Nokogiri::XSLT method would cause ruby to segfault after all tests were complete,
which is really just annoying, since it did not block the tests from actually running.
human accepts an optional xslt to use for formatting
Document the custom xslt option in the README