Initializr library

This commit splits the single file script in a library that can be
released and published to the central repository.

This allows anyone to start its own initializr instance with a
proper configuration file and the following simple script:

package app

@Grab('io.spring.initalizr:initializr:1.0.0.BUILD-SNAPSHOT')
class InitializerService { }

The integration tests have been migrated so that they run as
part of the build. Additional tests have been added to extract
the content of the generated archive and assert the directory
structure as well as the content of key files.

Fixes gh-20