Change how ComponentGroup.merge works

Now ComponentGroup.merge is guaranteed to return the original component group.
This is a lot more convenient for making standard chart templates, as it turns out.
Update unit tests.
Add StandardChart template in chart.ts
Update commit-chart.js to use new templating. It is shorter now :)
Incorporate Justin's suggestion: Label string can be overwritten
Merge pull request #230 from palantir/standard-chart

Standard chart