readme: Use ``` for syntax highlighting

Also used HTTPS for code.google.com.
readme: Add NPM version badge
Trim any trailing whitespace in files
Drop support for node.js 0.4 and 0.6

They'll probably stil work, but they're quite old. Let's ensure
officiall support 0.8 and 0.10 for now and also continously
integrate these via Travis CI.
test: Use proper exit code when tests fail

Locally npm-test was giving no sign of failure. Travis CI was
all-green as well.
test: Log details about which assertion failed

Previously it was outputting only the test name:

✓ flips floats
✓ does not flip URLs when flags are off
✗ flips URLs when flags are on
✓ flips padding

Now:

✓ flips floats
✓ does not flip URLs when flags are off
✗ flips URLs when flags are on
AssertionError: "background: url(/foo/bar.right.png)" == "background: url(/foo/bar.left.png)"
	at Object.tests.flips URLs when flags are on (/Users/krinkle/Development/cssjanus/cssjanus/test/cssjanus.test.js:251:10)
	at Object.<anonymous> (/Users/krinkle/Development/cssjanus/cssjanus/test/cssjanus.test.js:541:13)
✓ flips padding