Issue #287292 by almaudoh, mr.baileys, drewish, Berdir, znerol, boombatower, dawehner, jpetso, floretan: Add functionality to impersonate a user

- Patch #353458 by quicksketch, drewish, jpetso, sun, noahb, aaron, chx, mikey_p, dhthwy: hook_file_references() was not designed for a highly flexible field storage.

- Patch #462310 by jpetso: generate valid XML and make sure the tests work when clean URLs can't be enabled.

#142995 by dopry, drewish, quicksketch, jpetso, and flobruit: Adding hook_file_X(). This is an enabler of lots and lots of goodies. See CHANGELOG.txt for more. Awesome work, guys. :)