EZP-21562: Create synchronized repositories for API & SPI

http://jira.ez.no/browse/EZP-21562

Adds the required composer.json files to create the dedicated subtrees repositories: `eZ/Publish/API` to http://github.com/ezsystems/ezpublish-api and `eZ/Publish/SPI` to http://github.com/ezsystems/ezpublish-spi.

The subtree splitting is done every 5 minutes in a crontab with this procedure:

### ezpublish-api
```bash
git clone git@github.com:ezsystems/ezpublish-kernel.git
git remote add ezpublish-api git@github.com:ezsystems/ezpublish-kernel-api.git
git filter-branch --tag-name-filter cat --prune-empty --subdirectory-filter eZ/Publish/API master --tags
git push ezpublish-api master
```

### ezpublish-spi
```bash
git clone git@github.com:ezsystems/ezpublish-kernel.git
git remote add ezpublish-api git@github.com:ezsystems/ezpublish-kernel-spi.git
git filter-branch --tag-name-filter cat --prune-empty --subdirectory-filter eZ/Publish/SPI master --tags
git push ezpublish-spi master
```

## TODO
- [x] Create the repositories (http://github.com/ezsystems/ezpublish-api, http://github.com/ezsystems/ezpublish-spi)
- [x] setup the scripts
- [x] Create the packagist projects, setup the github hooks
- [x] Add dedicated README.md to the subfolders