Fixed validInteger to actually validate full input and always return type integer

Had problem with profiles, who had numbers in screen names like $twitterApi->users->show('JuicyBurger661') would give me 404, because request would look like: /1.1/users/show.json?user_id=JuicyBurger661 (see Twitter::createUserParameter)
Added tests for usersShow, $id can be screen_name or user_id (detects with validInteger)
Merge pull request #14 from mlamp/patch-1

Fixed validInteger to actually validate full input and always return type integer
Merge branch 'hotfix/14'

Close #14