Begun to work through recording seller-vendor-etc consensus.
Seller/vendor fixes, towards https://github.com/rvguha/schemaorg/issues/88
See https://www.w3.org/wiki/images/7/7e/ProviderSellerVocabularyRe-DesignProposal.pdf
Draft implementation detail overview: https://www.w3.org/wiki/WebSchemas/SellerVendorCleanup#Implementation_Details

This needs checking and domains for 'broker' confirming. Also examples.txt will need updates.
Merge branch 'master' into sdo-sellerbroker

Conflicts:
	data/schema.rdfa
'broker' property is expected on the Order type.
Merge branch 'master' into sdo-sellerbroker
Improved display of supercededBy information.
Previous code assumed a property superceded only one other.
Added supercedes_all to the API, and use it in per-type and
per-property pages. Should add caching in future, but the HTML
is cached so proceeding as-is for now.
seller is a suproperty of participant (esp. as we replace 'vendor')
Noted "and flights" re carrier. Added seller to Flight.
Added "Use 'provider' to represent carrier" to Flight.
Updated example to use 'provider' instead of 'carrier'.
Merge pull request #89 from danbri/sdo-sellerbroker

Fixes around seller, broker, merchant vocabulary to reduce variation and confusing.

Addresses https://github.com/rvguha/schemaorg/issues/88 - see issue tracker for backstory.