#249 Add PHPUnit "diff" output when exceptions are PHPUnit assertions

(it would be better OOP if PHPUnit added this to their exception message, of course)
Remove some now unnecessary PHPUnit exception formatting calls from FeatureContext
#249 remove redundant class_exists test
#249 review markups from @stof on pull req #250
Merge pull request #250 from RichardBradley/develop

#249 Add PHPUnit "diff" output when exceptions are PHPUnit assertions