Change PSCI module name to $PSCI (https://github.com/purescript/purescript/issues/530)

This fixes https://github.com/purescript/purescript/issues/530, and also resets the prompt to display `>` when the input string is empty. Multi-line commands still work, but one won't start unless there's a partial command being parsed (i.e. you won't have to press ctrl+d in order to get the prompt to come back).

I can remove the prompt thing if necessary, but I don't think it causes any issues and seems like it might be the desired behavior.