Quit after closing DevTools

[mattdesl]

Currently, if you close the DevTools window, the process continues to run since the main browser window (by default, 0x0 in size) is still showing.

I can hook into the following to determine when the DevTools window is closed, although it should only be done when the user does not specify a --frame size.

mainWindow.once('devtools-closed', quitHihat)

Adding this line causes problems on OSX when node-integration: false (causes Electron to crash). It also causes issues on Ubuntu, see #19.