move the core metrics lib into a separate package

[necolas]

should be a dependency of the grunt plugin, that way other people can use or wrap the metrics module without all the grunt baggage.

[phamann]

This already exists in the flavour of:

• CSS Metrics

which in turn uses:

• CSS Parse

I have merely wrapped Grunt around these (albeit re-wrote a bit of CSSMetrics lib to play nicely. Hence why its not included as a dependency).

[necolas]

I see. Any interest in up-streaming the changes to CSSMetrics?