Add Contribution guide, update Read Me with link
Update Contribution guide
style fixes
Expect stemcell to be placed at agreed location with stemcell_builder
Fail immediately if stemcell did not get created
DRY knowledge about whether micro or basic stemcell is being built
DRY spec setup and prefer more realistic literals in assertion
Explicitly require version arg

It turns out STEMCELL_VERSION was being set to Jenkins's BUILD_ID but
this was always being overriden by the BUILD_NUMBER passed through as an
argument to Rake. This change makes it explicit that the latter approach
is the only valid approach.
Invert builder/environment relationship

To conceal the latter from rake task