Move to Node.js' assert.js

[larsgw]

PRO

• A dependency less
• No more issues with 'use strict' (Browser test build #55, 7c475b6)
• Same functionality
• Supported by browserify
• Maintained by the Node.js people themselves (expect.js hasn't been updated since 2015)

CON

• A system library (but does that really matter?)
• It takes time to switch (especially since it's assert-style, not expect-style)

[larsgw]

A lot of these are converted while rewriting the tests for the new repository: citation-js/citation-js@57ba9ff#diff-098f6bcd4621d373cade4e832627b4f6. Only the core package tests are still expect.js.