Use Chai instead of node's assert

This will be easier to use for browser tests.
Change assert.equal to assert.strictEqual
Add Karma integration browser tests

Just runs the mocha JS tests for now. Still need to add a preprocessor
to automatically build C code.
Add Travis support for karma

Using suggestions from here:
https://github.com/karma-runner/karma/issues/1144

This is currently using chromium, which probably does not have NaCl.
I'll likely need to download a prebuilt binary from the Chrome
waterfall.