Path suffix

Hi,

Some APIs need a suffix like `.json` to get the right return format (ignoring accept headers...) so I added a string option to `Api()` and a `PathSuffix string` field to `ApiStruct` which is appended to `url.Path` in `Resource.do()`. I also added tests to check that query parameters are appended correctly.

How do you feel about this?

(p.s. travis fails because of some inconsistencies with the examples and my fork...)