Consolidated resolution. Fixes #413

This adds a new setting key called updateOptions, which can enable
consolidated resolution for update task.
The consolidated resolution automatically generates an artificial
module descriptor based on the SHA-1 of all external dependencies.
This consolidates the Ivy resolution of identical Ivy dependency
graph across multiple subprojects.

This is how it's enabled:

    updateOptions := updateOptions.value.withConsolidatedResolution(true)
Merge pull request #1454 from sbt/wip/resolve-consolidation

Consolidated resolution