Support graphql variables in Go client and fix small bug in UI

client/client.go

cmd/dgraph/main.go

cmd/dgraph/main_test.go

dashboard/build/asset-manifest.json

dashboard/build/index.html

dashboard/build/static/js/main.7a45a5b6.js

dashboard/build/static/js/main.ad984dfc.js

dashboard/src/containers/Helpers.js

gql/bench_test.go

gql/parser.go

gql/parser_test.go

protos/graphp/graphresponse.pb.go

protos/graphp/graphresponse.proto

query/common_test.go

query/query_facets_test.go

query/query_test.go

wiki/content/clients/index.md

wiki/content/query-language/index.md