Update assets

[njgheorghita]

Some changes addressed in this pr.

• Updated example contracts to newest version of solidity (contract assets in their respective packages still need updating to reflect these changes, which is coming in a future pr)
• Minor bug in export markdown script
• Updated use cases docs to reflect v3 changes.
• Added a test case to detect invalid manifest_version fields in v3 manifests.
• Added an example (simple-auction) to show how the ethpm spec can be used for a compiled contract's metadata output.

[njgheorghita]

closing this pr, to break it up into smaller prs