[in progress] Ditch browserify Buffer polyfill

Ditching the full browserify Buffer polyfill by explicitly using Uint8Array in browsers cuts down the browser build size from 43KB to 11.5KB (almost 4 times). Closes #8

Still need to polyfill some Buffer methods, but they shouldn't be big:

- [ ] readUInt32LE
- [ ] readUInt64LE
- [ ] writeUInt32LE
- [ ] writeUInt64LE
- [ ] write (for writeString)