Fix for Rickshaw GitHub Issue #89 

This pull request fixes up the Legend class so that it can properly respond to series that are being dynamically added/removed. I modified the extensions.html example so that it (quickly) adds the series one at a time to the graph. This will hopefully expose any non-dynamic problems in the future.

Tests conducted:
- Extensions.html thoroughly tested by manual operation.
- Exercised Highlight, Toggle and Order behaviors.
- Executed nodeunit tests.