Move latest stemcell detection into stemcel environment
Get infrastructure from stemcell to avoid dependency on environment
StemcellBuilder returns path to new stemcell (better encapsulation)
Decouple StemcellPublisher from StemcellEnvironment
Use stemcell path provide by builder

* Allowed removal of stemcell filename guesswork in StemcellEnvironment
* StemcellEnvironment no longer acts as a middleman for publishing
Rename, stemcell_path is more accurate
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
Add azure provider for blobstore_client.
Use VCR for azure offline testing.
Move vcr gem to blobstore_client gemspec
Azure functional spec changes.
Fix rubocop warnings