Suppress JRuby 1.7.x 'warning: instance variable $variable not initializ...

...ed'

JRuby 1.7.x complains about instance variables not being initialized:

.../threadpool-0.1.2/lib/threadpool.rb:30 warning: instance variable @terminated not initialized
.../threadpool-0.1.2/lib/threadpool.rb:27 warning: instance variable @running not initialized
.../threadpool-0.1.2/lib/threadpool.rb:28 warning: instance variable @finished not initialized
.../threadpool-0.1.2/lib/threadpool.rb:212 warning: instance variable @pipes not initialized

This change gets rid of these by initializing them upfront.