Support `package.$schema`

[peterdesmet]

CHANGELOG: https://datapackage.org/overview/changelog/#packageschema-new

• Requires #225
• Check $schema
  • If not provided, look for profile
  • If no profile, assume v1
  • If v1: investigate what effect is (likely small)
  • If v2: interpret new properties (likely none)
  • If v2+: warn that it may not understand all properties
• Update implementation
• Update documentation and mentions of profiles
• Update tests
• Update examples