Fix issue when building on Windows.
Tweak error messages.
Only remove `root` when needed.
Fix issues when the siteRoot and and url don't have slashes.

Conflicts:
	tasks/sitemap.js
Clean up package.json.

* bin/grunt-sitemap isn't really useful so remove it
* only install the needed files
* bump grunt dependency
Now, the "homepage" property should be read properly.
Improve README.md.