Replace simple-sha1

[stefandesu]

• [ ] Mapping identifiers use SHA-1 which is broken for any cryptographic purposes. But since we're not concerned about security aspects when it comes to mapping identifiers, it should be fine to continue using SHA-1 (and we should because some applications depend on the identifier to stay the same). Maybe we could add a note about this to README.
• [ ] Instead of having another dependency (simple-sha1), we could add a utility method that creates SHA-1 hashes (for example this one). This should also reduce the bundle this when we finally offer it. (#19)

[stefandesu]

It seems also that all modern browsers have a way to create SHA-1 hashes, see this answer on SO. But then we would need to add some code which can differentiate between browser and Node.js. Also, we should in both cases make sure that the hashes are definitely the same as the ones the current library creates.