Support other grant types aside from 'password' in oauth2.requestToken method

[TheTourer]

Currently, it appears that the only way to request an authorization token using this SDK is by using grant_type: 'password' and passing the appropriate credentials.

When using 3-legged OAuth2 and SignNow's login portal, external to a 3rd party application, an authorization code is returned as a query parameter in the redirect, and cannot be used with this SDK's OAuth2 requestToken method to fetch the proper authorization token response. In its place, a basic HTTPS call must be used, nullifying the point of using this SDK for auth purposes.