Allow providing a custom request factory

[andig]

This PR allows customizing the requests sent for APIs that require additional headers.

Replace #47, #48, #49, #64, #68.

This is not the most beautiful solution but offers the highest amount of customizing the request with the least amount of added lines of code. Making the request factory a private member and optional NewClient parameter would be a further option.

/cc @dmitshur

[fi0]

how do I provide my own request factory?

[andig]

I've kept it simple. After creating the client you would simply replace client.RequestFactory with a custom func(method, url string, body io.Reader) (*http.Request, error).

[andig]

Ping @dmitshur