Make tokenlimiter capable of getting rates from a request header

Ratelimiting middleware can be configured to retrieve Rates to be applied to requests. To do that one needs to pass a **ConfigMapper** function in an **Options** struct to the **NewTokenLimiterFromOptions**. The function should map an http request to a JSON string that configures token buckets for it. There can be configured several token buckets for a request. A request will be allowed by the middleware if all token bucket allow its consumption. The JSON token bucket config should look like this:
```json
[{"PeriodSec": 1, "Average": 10, "Burst": 20},
 {"PeriodSec": 60, "Average": 100}]
```
> If you dislike the idea with factories so much, I suggest just making methods Update and New get a []BucketSpec"

I don’t' dislike it, it's just that this library should not worry about the life cycle of the updates, as it all handled by Vulcand, so yes, let's just use `[]BucketSpec` as a parameters to Add, Get, Delete

I suggest just making methods Update and New get a []BucketSpec.