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.

Because STS hardcodes those IDs, a new (deprecated) property on type
has been introduced to keep track of them. When serving the legacy HTML
page that STS parses, those ids are used instead.

Fixes gh-39
Proper exception management

This commit updates the service to throw dedicated exception types for
common user error and associates proper HTTP status codes to them.

Fixes gh-38
More metadata for types

This commit adds a 'tags' attribute to each type. Two tags are currently
available:

1. build defines the build system that should be included in the project
(maven and gradle)
2. format defines the format of the project (build for the build file and
project for a project archive)

When a type id is specified, the build tag is used to figure out which
build system should be used. If no type is specified we fallback to Maven
as we were already doing.

Fixes gh-43