Allow customisation of the spec filename

This allows projects to support their own file naming scheme for the JavaScript
specs. For example my-module-spec.js or module.spec.js. This is done through
the new spec_matcher config property.
Update spec_paths to use === method on spec_matcher

This allows any object responding to === to be used as a matcher.
Update configuration section of the README

Now contains information on using the spec_matcher config option.
Merge pull request #96 from aron/spec-file-extentions

Allow customisation of the spec filename