add exometer_alias.erl

From exometer_alias.erl
This module implements an alias registry for exometer metrics. An alias can be either an atom or a binary, and maps to an entry+datapoint pair. The registry is an ordered set with binary keys, enabling straight lookup, prefix match/fold and regexp fold.

The purpose of the registry is to support mapping of 'legacy names' to exometer metrics, where the legacy names don't conform to the exometer naming standard.