Build with Node, part 1: run Ninja via npm scripts.

This change adds npm scripts for the major build tasks, providing equivalent
functionality to the existing Make targets:

- prepublish: ensure deps are downloaded and build file generated
- build: build default (spf.js)
- build-spf: build spf files (spf.js, spf-debug.js spf-trace.js)
- build-boot: build bootloader files (boot.js, boot-debug.js, boot-trace.js)
- build-dev: build development file (dev-spf-bundle.js)
- build-tests: build test runner and tests
- build-demo: build demo app
- build-all: build all
- dist: create distribution files
- test: run tests
- lint: lint files for style/formatting
- fix: automatically fix style/formatting of files
- start: run demo app
- clean: delete built files
- reset: delete built files and downloaded deps (get to newly-cloned state)

This change also updates the Travis CI configuration to use the above.

Progress on #7.