feat(utils): Add $exists method to AFUnwrappedSnapshot

[davideast]

Closes` #300. Also closes #464.

Exposes the DataSnapshot#exists function to the AFUnwrappedSnapshot.

af.database.object('myKey').subscribe((obj) => {
  if (obj.$exists()) {
    // object exists
  } else {
   // object does not exist
  }
});

[katowulf]

fyi - this doesn't look like part of the $exists impl. This may not generate sufficient release notes from the commit message.

[davideast]

I can make sure the changelog says the right thing.

[katowulf]

One quick comment about the content of the commit, otherwise lgtm.

[sean-hill]

Can this be available within the template also? Something like:

<pre>{{ (item | async)?.$exists }}</pre>

[alx-andru]

@sean-hill I like the idea! +1