Make user type more flexible

[ForbesLindesay]

The API seems to accept any JSON Object for setUserContext. By making it one method instead of two separate methods, we can also make it more flexible. Where before a wrapper would need to do:

function setUser(user?: User) {
  if (user) {
    Raven.setUserContext(user);
  } else {
    Raven.setUserContext();
  }
}

This type definition allows for:

function setUser(user?: User) {
  Raven.setUserContext(user);
}

Before submitting a pull request, please verify the following:

• If you've added code that should be tested, please add tests.
• If you've modified the API (e.g. added a new config or public method), update the docs and TypeScript declaration file.
• Ensure your code lints and the test suite passes (npm test).

[kamilogorek]

Isn't this the same as just setUserContext(user?: Object)? I'm not very familiar with TS.

[kamilogorek]

Could you also please rebase your changes?