add support for authentication plugins.#552

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craiggwilson wants to merge 2 commits intogo-sql-driver:masterfrom

craiggwilson:auth_plugins

Commits on Mar 17, 2017

add support for authentication plugins.
craiggwilson
committed

Commits on Mar 21, 2017

added support for closing an authPlugin for plugins whose resources need to be cleaned up.
craiggwilson
committed