Make the getFingerprint and getCustomFingerprint method asynchronous by adding callback functionality

[dsmontoya]

getFingerprint and getCustomFingerprint should work something like this

client.getFingerprint(function(fingerprint){
  console.log(fingerprint);
  console.log(client.getBrowser());
});

client.getCustomFingerprint(dataInAFormat, function(fingerprint){
  console.log(fingerprint);
  console.log(client.getBrowser());
});

Or maybe returning a promise...