Support third party development of ASN annotator

[gfr10598]

I've created branch "api" (https://github.com/m-lab/annotation-service/tree/api) to tweak the api to make addition of more annotators a bit simpler. The api change allows us to compose annotators, e.g. to combine an IPv6 and IPv4 annotator into a single annotator that handles both, or a Geolocation and ASN annotator into one that handles both.

In addition to an implementation of the annotator function, we also need a loader func:
func (string) (api.Annotator, error)
It takes a string filepath to a google cloud storage object, and loads the corresponding annotator. The loader should use the filepath as a key, but may load more than just one file.

[gfr10598]

Apologies for the current state of the repo. We are in the middle of both refactoring to improve the overall code structure and quality, performance improvements, and introducing a cache for multiple datasets. The 'prototype' branch contains a working prototype with many of these changes, which are gradually being merged into master as we add munge the code into a form we are happy with.

[pboothe]

https://www.caida.org/data/routing/routeviews-prefix2as.xml is where you can find the AS data

[gfr10598]

FYI, I've also created m-lab/analysis#2
It looks very straightforward to get histogram buckets as per the query I've added there. I think that will, in the end, be much more efficient that the current approach.

Feel free to add questions or clarifications in that issue.