Default thread pool size

This fixes issue #154.

I've included two separate, but related commits in this pull request.  The first just changes the default thread_pool_size when -j/--jobs is not specified on the command-line to 2.  The second also changes the thread_pool_size when -j is specified with no value so that it uses an "unlimited" number of threads.

The rationale is that an unfamiliar Rakefile might use MultiTasks that result in too many jobs for a system.  With this change, an unsuspecting or forgetful user of the Rakefile won't get burned by this when just typing "rake" on the command line.  

The second commit allows a knowledgeable user to obtain the old default behavior of "unlimited" threads without having to specify a numeric argument to -j/--jobs.