Use shared example group for NIO selectables
Use shared example group for NIO selectables

This approach allows tests for NIO selectables to be reused. The tricky
part still involves setting up example objects for the various tests,
which is broken apart into different let bindings

tpetchel/nio4r,frsyuki/nio4r,celluloid/nio4r,marshall-lee/nio4r,frsyuki/nio4r,celluloid/nio4r,tpetchel/nio4r,frsyuki/nio4r,tpetchel/nio4r,marshall-lee/nio4r,marshall-lee/nio4r,celluloid/nio4r