Add support for rate limiting

[ankushs92]

Every class can extend a RateLimit class, which can have fields int rateLimit and int rateLimitRemaining .

These fields can then be accessed as :

final Linode linode = api.getLinodeById(linodeId);
final RateLimit rateLimit = linode.getRateLimit();

final int limit = rateLimit.getLimit();
final int remaining = rateLimit.getRemaining();
final int reset = rateLimit.getReset();