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

initializr-service/application.yml

@@ -90,19 +90,19 @@ initializr:
           id: remote-shell
   types:
     - name: Maven POM
-      id: pom.xml
+      id: maven-build
       default: false
       action: /pom.xml
     - name: Maven Project
-      id: starter.zip
+      id: maven-project
       default: true
       action: /starter.zip
     - name: Gradle Config
-      id: build.gradle
+      id: gradle-build
       default: false
       action: /build.gradle
     - name: Gradle Project
-      id: gradle.zip
+      id: gradle-project
       default: false
       action: /starter.zip
   packagings: