Bigtable RowMutation should allow passing of a Mutation

While looking at PutAdapter, I realized that I'd like to create Mutation objects, and then later choose to create either a RowMutation or a BulkMutation depending on context. RowMutation needs a construction method that takes a Mutation (BulkMutation already has this).

Also, it would be great if we could pass in a TableName object instead of constructing one every time.