Improve the time complexity of `get` and `exists` methods from O(N) to
O(1).

This change affects performance of the following functions:

* Load time of program.json
* findByNetworkIdAndReplace

New private member variable `_itemMap` has been introduced, which maps
program IDs to their associated ProgramItem objects.

The `Map` class keeps insertion order.  So, we expect that the order
of program items in new `Program.items` is the same as the old one,
hopefully...

Dockerfile has been updated because it didn't work.  In addition, the
inconsistent line endings in Dockerfile have been fixed.

docker-compose.yml has been added for test purpose.  A data volume
which mounted at /usr/local/var/db/mirakurun is created automatically
when running `docker-compose up`.  Use `docker-compose down` with the
`-v` option if you want to remove the data volume at the shutdown.  Or
remove it manually using `docker voluem rm`.

This reverts commit fbc67e1.