Move main.js to lib/yammer.js

In preparation for splitting up yammer.js into multiple modules for each
different resource type.
Make Yammer the root export from yammer.js
Ensure Yammer is always an instance

Rather than requiring the constructor and instance to be defined
separately this allows us to use the popular terse instantiation:

    var yammer = require('yammer')({
        access_token: 'topsecret'
    });
Remove OAuth signature generation

Bearer tokens are used for authenticating requests rather than
OAuth signatures (see: http://developer.yammer.com/authentication/).
Parse JSON responses with request

Make use of the built-in JSON response parsing in `request` rather than
using our own response body parser.
Add Danny Amey to contributors
Remove json parse test

The built-in JSON parsing in request is now being used, so there's no
need to test this explicitly.
Remove Yammer.prototype._oauth

The authentication header is now a simple bearer token, so this commit removes
the extraneous _oauth method.
Make json the only API output option

Removes the trailing `.` and the option to specify a non-json response format.
Merge pull request #2 from dannyamey/master

Code removal and clean up