Allow better grouping of messages

If the message string contains state or time-specific data, Opbeat will
not recognize multiple errors as belonging to the same group, since the
message text differs. To group these kind of messages, send the message
as a parameterized message:

client.captureMessage({
  message: 'Timeout exeeded by %d seconds',
  params: [seconds]
});
Update readme with correct method fingerprints
Update readme with more advanced usage
Add TODO