Fix GitHub OAuth issue by adding a user agent to the file_get_contents options

[LIne 250 of lib/class.xpathr.php](https://github.com/alpacaaa/xpathr/blob/2897b4b8a13f6e7b78522a6ca2c41189ccb6bb81/lib/class.xpathr.php#L250) was throwing the following error on authentication:

    file_get_contents(https://api.github.com/user?access_token=sometokenhiddenhere): failed to open stream: HTTP request failed! HTTP/1.0 403 Forbidden

The changes described in a [similar issue for the codeigniter-oath2 repository](https://github.com/philsturgeon/codeigniter-oauth2/issues/57#issuecomment-29306192) were adapted to solve the issue.