Separate decryption from authentication

[qgustavor]

Currently the decryption function also do authentication. It's good because it makes using the library simpler. In the other hand it's not possible to do authentication checks on partial downloads, only in fully downloaded files.

So the proposal is separating internally decryption from authentication, keep the current functions doing that they already do and export an authentication function. Clients using partial downloads as a continue download mechanism (like megajs-cli) can verify the download after it ends using that function. They also can offer the option to check the MAC of some already downloaded file, allowing users to check if a file was already downloaded.