remove spec_helper from default backtrace exclusions

Also moved examples for backtrace cleaner defaults to backtrace cleaner
spec (from configuration spec).
use a guard to clarify the decoupling of in/exclusion patterns from full_backtrace
docstring improvements
tweak docs for configuration.backtrace_(in|ex)clusion_patterns
merge exclusion patterns using Regexp.union

Per benchmarks, this is 2 to 3x faster.
be true or false (v truthy or falsey)