Update Documentation for Provisioning API

Update documentation for Companies, Source Trackers, endpoints.
Sure.

> On Jun 10, 2014, at 4:25 PM, Elliott Wood <notifications@github.com> wrote:
> 
> In objects/companies.md:
> 
> > +      "created_at": "2012-06-20T19:18:48Z",
> > +      "disabled_at": null,
> > +    }
> > +  ]
> > +}
> > +```
> > +
> > +## Retrieving a Single Company
> > +
> > +Returns a single object describing the company with the requested `id`.
> > +
> > +### API Endpoint
> > +
> > +| Method | URL                             |
> > +| ------ | ------------------------------- |
> > +| GET    | /v1/companies/_company-id_.json |
> Can we use: /v1/companies/{company-id}.json here? The {} notation is the standard used by both Facebook and Twilio for describing ids within their REST APIs, and I'm a little worried the italics are easy to miss when sitting next to slashes.
> 
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