add an option to use summarized values from multiple sources

This is Librato specific
refactoring: extract points logic
support for composite metrics with librato
spec for composed metrics from aggregated sources, just in case
Use sum_means as it's the value that your looking for
Specify the value you want to use for your query

Default to :value, which was the previous default.
Document the other options in librato_metrics.rb
s/value_from/from

Seems simpler, we'll see
Only check compose if librato