Update authenticators

- [x] Add `PrepareHttp` and `PrepareCurl` for finer control in authenticators (enables basic and Windows auth)
- [x] Re-introduce `HttpBasicAuthenticator` and remove `Username` and `Password` from `WebClient`
- [x] Add `WindowsAuthenticator`
- [x] Resolve #9 with proxies not being used in `GoogleAuthenticator`
- [ ] Update all authenticators for Mac support