Update english.php

Added "Submit" text to language file for buttons.
Update contact.tpl

Changed button from "Continue" to "Submit"
Update contact.php

Changed submit button language call.
Update return_form.tpl

Changed return form button from "Continue" to "Submit"
Update return.php

Changed language call for return form submission button from "Continue" to "Submit"
Merge pull request #1705 from OpenCartAddons/patch-1

Changed "Continue" To "Submit" for Contact Us and Return Forms