Multi metrics

Motivation:

We want to be able to specify multiple measurements on a single line so as not to have to pay the (albeit low) overhead of writing to stdout. However, we don't want to take every k=v under the sun. L2met has always forced you to think about the things your are measuring and this feature does not regress in that regard.

Example:

```
echo 'measure=hello val=10 measure.world=10' | log-shuttle
```

This will result in 2 buckets:

bucket1 = {name=hello, vals=[10], ...}
bucket2 = {name=world vals=[10], ...}

Thus you can measure multiple things provided the key is prefixed with `measure.`.