Add Building Block section

[cmheazel]

The mod spec is incoherent without a building block model which is consistent with the rest of the specification model. Add a section which defines a building block as an integral part of the modular specification.

[cmheazel]

See #44
Using this approach the building block model can mirror the Standardization Target Types. Standardization Targets and Building Blocks form a single coherent architecture. No new SWGs would be chartered unless their proposed Standardization Target Types are logical extensions of this architecture.