Add buffers argument to comm open/close

[vidartf]

Adds an argument buffers to services.kernels.comm.Comm.open()/close(). It is passed through to the send_shell_message similarly to what is done in Comm.send().

Fixes #2730.

[stonebig]

#2730 ?

[takluyver]

Thanks. Should CommManager.prototype.new_comm also gain a buffers parameter and pass it through to comm.open()?

[takluyver]

@minrk @gnestor I've marked this as 5.1 since it's a fairly simple change that makes sense to me, and seems unlikely to have adverse effects.

[vidartf]

@takluyver I added the buffers argument to new_comm as well. I added it at the end to retain compatibility with old signatures, even if the grouping of arguments then gets a little different.

[takluyver]

Thanks, I think that's the right thing to do.

[gnestor]

Thanks @vidartf 1