Improve cookbook idempotency

[gildegoma]

Two modes must be available (probably via a boolean attribute):

• install only once: Don't re-execute android sdk update if components are already present (Related to #9 and #10)
• full update on provisioning: Do re-execute android sdk update to force updates (a.k.a re-installation)

Instead of a single guard, it might be interesting to specify an idempotent guard (file or directory) per SDK component (e.g. folder build-tools/19.0.3 for build-tools-19.0.3, etc.)

[gildegoma]

Maybe more work here, due to #14.