Fix #161: support save/load BiRNNBuilder
Fix #161: support save/load BiRNNBuilder


Former-commit-id: 3fa9d526e3ec23d22d6a1049988a648704f635aa
danielhers/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,clab/dynet,clab/cnn,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,xunzhang/dynet,clab/cnn,xunzhang/dynet,clab/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,chunyang-wen/dynet