Fix rule.remove and rule.metric

The data bound to rule was inconsistently either an integer or an object with
an integer attribute i.  This affected both rule.remove in that it tried to
remove a listener using this data, and the styling of rules drawn from
rule.metric which is used to draw rules at specific times to denote events such as deployments  (which is apparently undocumented, but super useful).  These are the divs classed as "metric line" in the code. This functionality was introduced here https://github.com/square/cubism/commit/c4d23ab64c41df3d337530ef4ef5ad268815a600

Attached is an example of rules drawn using rule.metric
![rulemetric](https://f.cloud.github.com/assets/419698/340969/918b21e8-9d61-11e2-848e-f4b9cf4d5cad.png)

This pull request picks up of of https://github.com/square/cubism/pull/40