Rename the `memberships` table to `takes`; #2282

`memberships` was a bad name for this table because it's ambiguous with
community memberships. Renaming to `takes` fits the usage we've arrived
at and is parallel to `tips`. Tips and takes! Membership is then an
emergent property of data recorded in the takes table.
minor change (make code more DRY)
add test for MixinTeam.get_members()
fix bug in MixinTeam.get_members()
Merge pull request #2287 from gittip/rename-memberships-table

Rename the `memberships` table to `takes`