Update Rakefile for newer rake version

The check_dependencies task no longer exists.
rake/rdoctask was removed and is not rdoc/task.
Add Gemfile
human accepts an optional xslt to use for formatting
Document the custom xslt option in the README
Generate gemspec
Merge pull request #1 from nilbus/custom-xslt

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