chore(package): use `npm run` to build and reduce devDependencies

reduce devDependencies as much as possible to gain dedupe-friendly dependency chain.
chore(package): remove gulpfile from module files
chore(power-assert): extract espower-cli module out

https://github.com/twada/espower-cli
chore(power-assert): fix sandbox examples
docs(README): refer to espower-cli
chore(package): apply dereserve on build
chore(travis): run CI on docker
Merge pull request #15 from twada/npm-run

use `npm run` for build