Add datapackage file

Documentation: http://data.okfn.org/standards/data-package
Initialize datapackage.json with MUST terms

Metadata: name
Resources: path
Add SHOULD terms to datapackage.json

Metadata: licenses, datapackage_version (could not find good
documentation on what version to use here)
Resources: name
Add MAY terms to datapackage.json

Metadata: title, description, keywords, last_modified, sources. Did not
use version and image.
Resources: format. No plans to add other optional fields, except schema.

Also updated title of README
Add maintainers to datapackage.json
Add schema: cartodb_id, created_at, updated_at
Update schema: code, verified, street, facing, code_prefix, longitude, latitude, geojson

* Noticed geojson is rounded.
* Almost 100% coordinates are in webmercator
Update fieldnames to text_on_sign and text_below_sign, and added to schema

Original field names: `details` and `additional_details
Add URL and values to schema

Not 100% sure this is allowed, but is not forbidden in the
[documentation](http://www.dataprotocols.org/en/latest/data-packages.htm
l) and extra terms like `foreignkey` are used
[here](https://github.com/datasets/gdp/blob/master/datapackage.json)
Merge pull request #5 from peterdesmet/datapackage

Format dataset as data package