Merge pull request #169 from upper/experimental/builder

builder/builder.go

builder/builder_test.go

builder/cache/cache.go

builder/cache/hashstructure/hashstructure_test.go

builder/convert.go

builder/delete.go

builder/errors.go

builder/exql/column.go

builder/exql/column_test.go

builder/exql/column_value.go

builder/exql/column_value_test.go

builder/exql/columns.go

builder/exql/database.go

builder/exql/database_test.go

builder/exql/group_by.go

builder/exql/hash.go

builder/exql/join.go

builder/exql/order_by.go

builder/exql/raw.go

builder/exql/raw_test.go