Store offline data in an array rather than in a hash

Store offline data in an array rather than in a hash

This has several reasons:

1) The hash (i.e. localStorage) used timestamps as keys. When doing
   automated testing, multiple events can happen to have the same key,
   which causes problems - see the next point.

2) Events from localStorage has to be sent to the web server in the
   order of their creation. This means that before sending, we had to
   convert the hash into an array and sort it according to the
   timestamp. This can be avoided if we use an array from the
   beginning.