Add filter for $templates_dir variable

Added a simple filter (`edd_templates_dir`) to give themes the option of setting their own template directory.
Helper function to get template directory name.
Moving edd_get_theme_template_dir_name() to template-functions.php
Added edd_get_theme_template_dir_name() function
Merge pull request #1255 from Studio164a/master

Add a filter for the templates directory.