Adds server support for difference.
Adds array arith to js and python drivers.
Adds tests for difference.
Adds docs for array difference.
Makes difference work on streams.
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Merge branch 'next' into merge_br

Conflicts:
	src/rdb_protocol/terms/rewrites.cc
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next

Conflicts:
	src/rdb_protocol/ql2.proto
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next