Made setUserAgent function that allows users to set user agent

You can now use a customer user agent, which is required to be set if you wish to use this as a bot for Reddit or something similar.

All you have to do to use it is:
```
var reddit = require('redwrap');
reddit.setUserAgent("My Awesome Bot/v1.0 by Conquerd");
reddit.r("wtf, function(err, data, r){
    //Do stuff with data
});
```