Add license

This addresses #3 by licensing the project under the Apache License, Version 2.0 - points for consideration:

1. Given it is hosted under the City Index organization, I have assigned copyright to City Index (only) right now, which might not be accurate.
1. As recommended by the Apache license, I have added the usual boilerplate license header to each source file; some argue that this isn't required, but it is a widely used best practice, and also helpful for license management.
1. The project seems to depend on [Blockly](https://code.google.com/p/blockly/) only, which is itself Apache licensed, but doesn't provide a `NOTICE` file, thus none is required here either (see [Overview of the LICENSE and NOTICE files](http://www.apache.org/dev/licensing-howto.html#overview-of-files)).

@fandrei, @mrdavidlaing - please review/adjust the copyright assignment in 1. in particular.