Fix bug in millisecond-to-string logic

[jugglinmike]

Since I needed to create a new test file for the ms module, I rounded it off by testing the entire module. I can imagine that these tests might be too low-level, so if you'd prefer, I can try writing integration-style tests instead.

Commit message:

When generating a string representation of a millisecond duration, use
the rounded value to determine if the unit should be pluralized.

This commit also introduces unit tests for previously-untested string
parsing logic belonging to the same module.

[tj]

i think this was fixed in https://github.com/guille/ms.js, we could copy/paste that over

[jugglinmike]

It looks like it was. Is there any reason not to include it as a module?

[jugglinmike]

I see why now: mocha's build process isn't currently set up to package NPM modules for the browser. I've opened a new pull request to address the underlying bug: #957

[tj]

yeah it's a bit hacky right now haha, pre-{browserify,component} era