CRM-789 Configuration: All civicrm.settings.php directory and URL values should handle values with AND without trailing forward-slash

Currently our code and documentation is inconsistent w/ regard to requirement for trailing slashes on file system paths and URLs.

1. Modify all 3 Config.php files (Core, Contribute and Mailing) to have consistent behavior for every setting which is either a URL or a file system path.
The code should check for a trailing forward slash and add one if not present. (We will document the file and advise users to include trailing - but the code should add one as it adds to the config object if it's missing.

2. Check all functions which use these config object settings - and fix them if they are NOT currently coded to expect a a value with a trailing .