Move JUnit runners to serenity_bdd package
SerenityRunner and SerenityParameterizedRunner now contain functionality, and Thucydides equivalents merely extend

Also move a number of helper classes into serenity_bdd package
Rename package in demo to serenity_bdd
Move junit finder classes to serenity_bdd package
Merge pull request #12 from mikezx6r/master

More package renaming