add custom prefix, refactor format and GithubHook string

Allows me to do stuff like:

```js
var githubhook = require('githubhook');
var format = require('util').format;
var github = githubhook({
  logger: console,
  prefix: function () {
    return format('[GithubHookkkk %s]', (new Date()).toISOString());
  }
});
```

```
[GithubHookkkk 2014-07-27T23:36:54.055Z] listening for github events on 0.0.0.0:3420
```