Auto-Enabling return key doesn't work with external keyboard

[ZevEisenberg]

This is kind of a corner case, but related to #7: if you set enablesReturnKeyAutomatically = true, it won't enable if you're entering text with an external keyboard. Of course, you're probably not seeing MMNumberKeyboard if you have an external keyboard anyway, but you could theoretically present it, I think. At least, you can in the simulator. I don't have an iPad with a physical keyboard handy to check, so feel free to close this if I'm just wrong.