Making 'get' and 'post' methods more intuitive

Sorry my english, this is a small commit. Actualy you need start with "/" and apply the ".json" extension in methods get/post's urls.

This does not intuitive:

```js
get('/users/show.json',{screen_name: 'exos'}, function (err, res) {
    ...
});
```

instead of presume a url not begun by "/" is a complete URL, I presumed opposite:


```js
get('users/show',{screen_name: 'exos'}, function (err, res) {
    ...
});
```

In my version is correct, and translate to ```https://api.twitter.com/1.1//users/show.json?screen_name=exos```

For compatibility the follows urls are accepted:

* 'users/show'
* '/users/show'
* 'users/show.json'
* '/users/show.json'

But, if an url start with http://, https://, somestring://  the url has considerate absolute. Ex 'http://anotherhost.com/users/show.nojson' is'nt translate.