feat(Storage): file removal ability.

[NathanWalker]

Adds the ability to remove a file:

removeFile

You can pass in remote file path to remove it.

  firebase.removeFile({
    // optional, can also be passed during init() as 'storageBucket' param so we can cache it
    bucket: 'gs://n-plugin-test.appspot.com',
    // the full path of an existing file in your Firebase storage
    remoteFullPath: 'uploads/images/telerik-logo-uploaded.png'
  }).then(
      function () 
        console.log("File removed.");
      },
      function (error) {
        console.log("File removal Error: " + error);
      }
  );

Docs updated. Android/iOS.

[NathanWalker]

This also dries up some code throughout all the file methods. 👍

[EddyVerbruggen]

Great work, Nathan!

I'll merge it and add the TS definition.

Thanks a bunch 👍