Update "seller" property from schema.org to "offeredBy"

[Dan Scott]

* Evergreen 2.6 and on (but only targeting master / 2.11)

Per the 2015-11-05 release of schema.org (http://schema.org/docs/releases.html#v2.2), the "offeredBy" property is now the preferred property to use for non-commercial entities (such as libraries) making an offer of an item for borrowing, sale, etc.

There is only one occurrence of the "seller" property in Evergreen's TPAC templates, so this is a relatively low-risk fix that would bring us back into compliance with best practices.

See http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/dbs/lp1608711_seller_to_offeredBy for the relevant commit.

[Ben Shum (bshum)]

Seemed like a bug fix to me, so I pushed it to master and backported to rel_2_10 and rel_2_9.

Thanks Dan!