Gauge for number of states with candidates

Hi all,

This tiny addition is currently the only difference left between the version of crawljax I use for my project and the current 4.0 branch. It would simplify further development if you merged this :)

The addition is a gauge to get the number of states with candidate actions. I use this to create live graphs of the development of total states and states with candidates over time.

@alexnederlof: have a good time in India, hopefully the coffee gets better..

-- Ivan