Alternate binstubs support

Builds on request #22 but could be reimplemented independent of it.

When using more than one ruby, "bundle install --binstubs" will continually be overwriting the binstubs put into bin/.

This patch builds on request #22 (Rakefile bundler awareness) and makes it possible to specify an alternative binstub directory at install and run-time. If using an alternative binstub directory, you can't run the tests with the :default task anymore, you need to use :runtests[rake] to specify which rake will be called (e.g., jruby-bin/rake if you used :setup[jruby-bin]).

This review is primarily about 39b5129; request #22 is about 62fa3d2.