Rename Report to Result

Improves semantics as a first step toward decoupling the measuring of metric
"results" and the formatting of those results into "reports" (or whatever).
Test that data yml is created during run.
Add basic formatters.
Create reporter to notify configured formatters.
Replicate current report behavior in HTMLFormatter
Remove Result methods that have been moved to formatter
Configure formatters.
Allow formatters/templates to use custom output.
Remove need for a formatter base class
Make output paths consistently relative to base.
Add format/out help descriptions.
Remove short options for format/out.
Use fakefs for run/formatter specs.
Restrict display_results specs to darwin platform.
Lazy initialize formatter output.
Dont use fakefs on rbx.
Select metrics for spec based on platform.
Require file and retry when loading class fails.
Update README with formatter documentation.
Eliminate ruby warnings.