Port configure script to Node, step 6: build demo app.

Progress on #7.
Merge pull request #199 from nicksay/build-with-node-6

Port configure script to Node, step 6: build demo app.
Port configure script to Node, step 7: lint JS files.

Progress on #7.
Merge pull request #200 from nicksay/build-with-node-7

Port configure script to Node, step 7: lint JS files.
Port configure script to Node, step 8: add shortcut targets.

Progress on #7.
Merge pull request #201 from nicksay/build-with-node-8

Port configure script to Node, step 8: add shortcut targets.
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.