NPM: Verify credentials upfront

Fixes gh-27
Closes gh-36
NPM: Use the package name to look up the npm owners

Follow-up to 8a71c2e which uses the Release.project property. For QUnit,
the npm package name is 'qunitjs', not 'qunit'.
Util: Release.exec() aborts on failure

Removed Release.git() as it no longer provides much value.

Fixes gh-22