implements refresh access token (session id) for soap-based (username/password) login

Hi @stomita,

We'd like to use your fantastic library for our MavensMate for Atom plugin, however, we require soap-based logins. This PR adds token refresh functionality for soap-based logins. If you accept this PR, we'll move forward implementing the Atom plugin with jsforce.

Feel free to reach out with any questions.

Cheers

Joe Ferraro