Fix documentation of require
Import documentation from docs.rubygems.org
Improve PackageTask tests

The previous tests did not invoke the rake task to ensure functioning

Also, add explicit directory tasks as it seems that sometimes the file
task does not imply them.  I'm unsure if this is a rake version issue or
not.
The pristine command downloads missing .gem files now
Clarify `gem pristine` documentation

The behavior of the command with respect to its arguments was ambiguous.

How to use the --extensions option was also unclear (See also #619).
Improve description of extension rebuilding

Remove plural references to "gem" as --all implies plurality for the
actions in the description.

Feedback from @postmodern