Provide pop-up lisitng keyboard shortcuts

Pressing the question-mark key (?) produces a pop-up listing the
keyboard shortcuts. Pressing q whilst this window has focus closes it.

I've provided crude translations for the additional ? and q shortcuts,
obtained from Google Translate, but they should be checked by native
speakers.
Mention shortcut to show shortcuts in README
Merge branch 'master' of git://github.com/jamesscottbrown/miniflux into popup
Improve help popup (pull-request)