Provide default value for uc_signup_add_cart_text .

UC_Signup provides a configurable option to change the "add to cart" text for signup enabled products to something more signup-related, like "Sign up for this event", however we don't provide a default value automatically.

We could provide one such as "Sign up to this event" which is mostly consistent with Signup module's language, however for sites where events are called something other than 'event', such as workshop, class or party, the word event may be appropriate. I suppose we the saved variable could include a token, such as "Sign up for this [content-type-name]" and we could provide a helper function to do the string substitution of the human-readable name for the content type, and also lower the first character to so we get, "Sign up for this event" rather than "Sign up for this Event". This may be overkill though.