This commit splits the feature of Spring Initializr in several modules:
* `initializr-generator` is a standalone library that is responsible for
generating projects based on a `File` directory. It has a minimal set of
dependencies and is not web-related
* `initializr-web` provides the web integration (project generation,
meta-data, etc)
* `initializr-actuator` is an optional module that can be added to
support project-generation-specific statistics
Closes gh-214
