Refactor project

[nunofgs]

This PR depends upon node-oauth-server#203 and introduces a cleaner koa-specific implementation that also allows for the OAuth model class to export generator functions instead of node-style callback functions.

Furthermore, the examples previously found in the node-oauth2-server repository were moved here since they're all dependent on Koa.

Koa-OAuth-Server

Introduces a new Koa OAuth server wrapper with the following changes:

• Added wrapping of model functions to support generator functions.
• Fixed OAuth token is now exposed in the Koa context's state instead of injected into the current context.
• Fixed objects retrieved from the model are no longer injected into the execution context.
• Improved handling of errors by simply copying the underlying response, instead of hard-coding the error, _errordescription and code properties.
• Removed express framework-specific response handling.
• Removed passthroughErrors in favor of always emitting the thrown error.

[ruimarinho]

Now that https://github.com/thomseddon/node-oauth2-server/issues/203 has been merged, we need to get this merged into master. Other fixes will follow soon.