divide Sproutvideo::Video.signed_embed_code for convinience

Sproutvideo::Video.signed_embed_code is very useful.

But if I want to get both HD and SD video signed url, 
I do not want to use the method to reduce API call.
Because Sproutvide API has a rate limit at 100 requests per minute. 

Sproutvideo::Video.signed_embed_code() is called get single video API in it, so we need to call API twice at one transaction.

So, I want to divide the method into two method. We can generate many signed embed code, but only one API call is needed.