Make multiple metrics on one line explicit.

The multi-metric support added as part of #27 considered anything matching `key=floatval` on the same line as `measure=foo` to be a metric in the form `foo.key` with a value of `floatval`.

This PR changes that a bit to make things more explicit: things matching `measure.key=floatval` on the same line as `measure=foo` will be considered to be a metric in the form `foo.key` with a value of `floatval`.

My motivation is cutting down on measurements sent to librato for metrics that won't be useful which translates primarily to lower cost.