Version bump to 0.4.0.
Add a --rubies-dir option for installing into /dir/$RUBY-$RUBY_VERSION.
Added --rubies-dir to the man page.
Updated the RVM/rbenv integration examples to use --rubies-dir.
Drop support for Ruby 1.8.x since it has reached End-of-Life.

* CVE-2013-4164 affects Ruby 1.8.x and there is no official patch.
  Enabling users to install and run 1.8.x at this point is irresponsible.
Update tests to not use Ruby 1.8.x.
Add --jobs flag for compiling in parallel.
Add documentation for --jobs option flag.
Merge remote-tracking branch 'havenwood/jobs' into 0.4.0
Renamed JOB_OPTION to MAKE_OPTS and define it as an Array.
Match the --jobs option parsing of make(1).

* Use extglobs to exactly match the -j[JOBS],--jobs[=JOBS] options
  and simply add them to MAKE_OPTS.
Renamed rubinius to rbx for consistency (closes #121).
Merge branch '0.4.0'