Avoid reaching Github API rate limit

[rafaeljusto]

As we are today using token bucket strategy to limit the number of requests and the Github API uses requests per hour strategy, we can fall into rate limit when running the scanner. To solve this we could:

• Change the token bucket parameters and sleep the go routines (until X-Ratelimit-Reset) in case that we reach the rate limit (status 403 Forbidden). The problem here is how to stop filling tokens in the bucket while in rate limit.
• Rethink our rate limit strategy to query Github API, maybe something controlled by Github HTTP response headers (X-Ratelimit-Limit, X-Ratelimit-Remaining, X-Ratelimit-Reset).

[rafaeljusto]

https://gist.github.com/garyburd/014b16f88b031e55f084

[rafaeljusto]

I decided to simplify the rate limit strategy. If we got a 403 Forbidden response from Github API, just wait for the next requests window informed in the HTTP headers response (X-RateLimit-Reset). Token bucket was removed.