Add a new BOM packaging

The BOM packaging provides a way for Maven to differentiate parent POMs from BOMs.  This is required in order to be able to reduce the consumer POM : BOMs' POMs can be reduced easily as all the build time information is irrelevant, however parent POMs must preserve all build time information.

In order to preserve compatibility, the BOM package will be translated into a POM packaging at publication time in the consumer POM.