Dm devdependencies

I've found these issues (npm install --dev fails):
* https://github.com/npm/npm/issues/6137
* https://github.com/npm/npm/issues/6043

--dev seems to install things recursively (1.4GB now !!), it is marked as deprecated.
No longer needed.

I've created a dm-devdependencies branch with:
* readme modified (prerequisites -> npm version constraint)
* readme modified (npm install, no more --dev)
* package.json (missing dev dependencies vinyl-source-stream, gulp fails otherwise)