support sending ApiKey as Basic Auth

[sclasen]

Unless I cant see quite how to do it, I think there should be a way to configure sending the api key as a proper base 64 encoded basic auth header, for those apis that work in that manner.

[rintcius]

I created a pull request in swagger.js that enables setting headers globally. See swagger-api/swagger-js#4

[ayush]

thanks @rintcius Also updated readme

[rintcius]

hi @ayush Maybe also mention that in order to use these headers this also needs to be set:

supportHeaderParams: true