Add btree_gin contrib module.

contrib/Makefile

contrib/README

contrib/btree_gin/Makefile

contrib/btree_gin/btree_gin.c

contrib/btree_gin/btree_gin.sql.in

contrib/btree_gin/expected/bit.out

contrib/btree_gin/expected/bytea.out

contrib/btree_gin/expected/char.out

contrib/btree_gin/expected/cidr.out

contrib/btree_gin/expected/date.out

contrib/btree_gin/expected/float4.out

contrib/btree_gin/expected/float8.out

contrib/btree_gin/expected/inet.out

contrib/btree_gin/expected/install_btree_gin.out

contrib/btree_gin/expected/int2.out

contrib/btree_gin/expected/int4.out

contrib/btree_gin/expected/int8.out

contrib/btree_gin/expected/interval.out

contrib/btree_gin/expected/macaddr.out

contrib/btree_gin/expected/money.out