feat(build): write packages to dist/ instead of build/

- in "dist-zip" and "dist-maxthon" write results to dist/
- add "clean-dist" task which cleans dist/, use it in "dist-*" tasks
- rename "clean" task to "clean-build"
- ignore dist/
feat(build): watch for changes and build continuously

- add "build" task which simply runs "scripts" task
- make "default" task watch for changes in source/ and call "build"
fix(build): in "clean-*" tasks retry fs.remove() until success

Being a workaround, this closes #58.