Double underscore in step method name causing stack overflow

[tmaierhofer]

if you have a step method name like my__step_method
it will cause an stackoverflow. (Because the closing double underscore is missing?)

[JakeGinnivan]

Damn.. that a bad bug

[jbergens]

I did a pull request (#225) with another fix that might make this a little better also. It should give an exception about an illegal example name.