Add support for asset creation

Based on KnpLabs/php-github-api#85 I have taken a first stab at support for creating assets.

I do not have that much experience using this library or its code style so feedback would be much appreciated.

The related test case passes but I have tried to create an actual asset using the following code:

```php
$client = new Github\Client();
$client->authenticate($username, $password, Github\Client::AUTH_HTTP_PASSWORD);
$return = $client->api('repo')->releases()->assets()->create('kasperg', 'release-asset-test', '0.1', array('name' => 'test.txt', 'content-type' => 'text/plain', 'body' => '123'));
```

This fails with the following exception. I am unsure why.

`Github\Exception\RuntimeException: You have reached GitHub hour limit! Actual limit is: 5000 in /Users/kasper/Code/php-github-api/lib/Github/HttpClient/HttpClient.php on line 138`

If we at some point decide to merge this, [the documentation](https://github.com/KnpLabs/php-github-api/blob/master/doc/repo/assets.md) will have to be updated as well.