Include ERb filename in backtrace
Fix command_guide task

For some reason this no longer works with RubyGems 2, or never did
Add default rake task, improve user-friendliness

Previously running `rake` would give you an error about a missing
rubygems checkout.  This no longer occurs unless you invoke the
rdoc_spec task.

Previously running `rake` would not do anything particularly useful if
you had the RUBYGEMS_DIR set.  Now it builds all the documentation and
style sheets and starts a jekyll server.