Allow non-Class-based metric naming.

Create MetricsBuilder with MetricName rather than Class[_].
MetricName.apply(Class[_]) for standard naming.
Merge branch 'non-class-derived-metric-names' of https://github.com/arosien/metrics-scala into arosien-non-class-derived-metric-names
Small tweaks to @arosien 's allow non-Class-based metric naming (#35).
By default do not build for Akka (fixes Travis build).
Allow override of InstrumentedBuilder's and CheckedBuilder's metricBaseName.