Corrects a fatal error:
- the slug being passed to get/set_site_transient is now being scrubbed to prevent "Data too long for column 'option_name'" errors

General Improvements:
- 'slug' setting has been moved to be the plugin directory name (less will be removed, see above, and more likely to be meaningful and unique
- adds 'plugin_file' setting to specify the file in which to find the Wordpress Plugin header
- when checking for new version number, if a version is found in the 'plugin_file' the 'readme' check is skipped
Issue #49:
- wraps class in ABSPATH and class_exists check to better protect against multiple plugins using this class