polishing
Generate proper project archives

This commit generates an archive name that is consistent with the
chosen artifactId for the project. This prevents all archives to be
named 'starter.zip'. If the custom artifact contains special
characters, these are encoded (spaces are replaced by _ which seems a
sensitive default)

Fixes gh-36
Update to latest boot release
Fix type id

This commit improves the structure of the type id as it may be used by
third party clients. The id now defines the build system and the nature
of the project.

Fixes gh-39
Add Content-Disposition to all endpoints

Prior to this commit, the browser was "guessing" the file name to use for
Maven and Gradle build files. This commit harmonizes the endpoints so
that they provide a Content-Disposition header with a preferred file
name.