minification build script

Build script to run it through Closure Compiler.

Stats:
- original: 5793 bytes gzip
- minified: 1700 bytes gzip

Procedure:
- `cd build && sh build.sh`

Output is build/output/auto-tabbing.min.js

Requirements to run
- UNIX-like (MacOS or Linux)
- basic tools (rm, mv, cat, cp, echo)
- curl
- perl
- bash (or similar, nothing too fancy)