Ignore errors loading rdoc and rdoc/task.

rdoc.rb is not included in Ruby 1.8, rdoc/task.rb is not included prior
to Ruby 1.9.2 and on Ruby 1.9.2, the built-in version of rdoc/task.rb is
not compatible with later versions of rake - causing a RuntimeError to
be raised when required.
Add task descriptions.
Add a default rake task.