OAuth1 RSA SHA1 signature support #203

Hi,

I started using this library to perform oauth1 with Bitbucket Server (previously Stash)

The code I needed was pretty much the same you have but with some slight changes in the signature, to use RSA with SHA1.

Would you be willing to accept my contribution with this code?
If yes, what would it be the best approach to support both signatures?

In sum it needs the header value to be changed and the signature code.
You can check the modified version here https://github.com/codacy/stash-scala-client/blob/ba4e55b0d58604b47c100111fdca627bb02d3d85/src/main/scala/com/codacy/client/stash/client/auth/OAuth1.scala