Fixes missing entropy in the uuid package.

math/crypto is seeded with 1 and thus will create predictable UUIDs. Because
amazon-instance and amazon-ebs in the same second when building both targets
the timestamp in front doesn't help either. See #552
Simplifies the implementation of common/uuid
Merge pull request #557 from zimbatm/unique-uuids

Fixes missing entropy in the uuid package.
Update CHANGELOG
Merge branch 'master' of github.com:mitchellh/packer into fix#536

Conflicts:
	CHANGELOG.md