add universal_url_prefix for APIManager

[elvismacak]

add a global url_prefix in the apimanager, so no need for user to specify url_prefix in each create_api method

[elvismacak]

test locally success, but failed remote in travis

[jfinkels]

Thanks for your contribution! I have implemented this in a slightly different way in 46c2c2d. I have left the behavior a little bit simpler than what you suggested here: a url_prefix in the create_api() method will simply override the value supplied in the constructor. To allow the more complicated "appending" behavior you suggested here, please open a new pull request with some justification for why that would be valuable.