Indented command hits error

[takluyver]

We previously allowed entered code to be indented, for convenient pasting from editors, web pages, etc. This no longer works in 0.11rc1: <space><space>a=1 gives an IndentationError.

I think previously we solved this by prepending if True:\n if the first line was indented, although I don't know if there's a neater solution.

[takluyver]

Added a message about using the %paste magic, which is as fixed as this is going to get for now.