Refactoring

A couple of things are going on here:

## main

This is the http receiver. As such, I moved most of the receiving logic out into a receiving package. I added a test for the receiver to boot!

## bucket

After explaining l2met internals to several folks, I found myself using a set of terms that seemed to resonate. The changes I made to the bucket code reflect these new terms and models. There now exists:

* bucket.Id
* bucket.Bucket

## store

One of the great things to come out of these changes is that redis logic is isolated to store/redis_store. Also, the store is now an interface. Consequently, I implemented a MemStore that can be used in testing.

## tests

I added receiver/receiver_test.go which not only tests the receiving path. But also highlights to how to the new code paths.

## data flow

I have taken great care in not changing the how data flows in & out of l2met. I wanted to preserve all of the performance tuning that we have been doing.