Added base to amd and created a new oauth2-bearer provider.

Hi,

I've added the base provider export to the AMD as you suggested in https://github.com/Vestorly/torii/issues/106 - works great.

I've also taken the opportunity to create an OAuth2 Bearer provider, as that was the reason for me needing to use the base provider in the first place. Thought it could be useful for others that required a token based response type, as per this other issue https://github.com/Vestorly/torii/issues/43

Hope thats OK for you, i'm new to creating PRs like this.