Handle error codes in Twitter::ClientError

Receiving a lot of different errors in an application, We started to look at errors code in twitter errors.
https://dev.twitter.com/docs/error-codes-responses

It seems logical to us to add them to the Twitter::ClientError class and handle them in our code.

That way instead of parsing the error message, we can compare error_codes in our code.

First PR here so don't hesitate to criticise my code and the way I handled that one.