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.