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
Add new iframe allowed resources
Remove PHP notice in PicoFeed
added dark theme v1.0
updated translation
Merge branch 'themes' of https://github.com/facciocose/miniflux
Replaced Host Address with Feed Title
Improve css for mobile version
Improve dark Theme
Enable theme on login page
Update readme
Change color of the alert box for midnight theme
And here is the Green theme !

There are still things to finish and fix, especially for the responsive
version (I'm not a responsive design guy), but nothing blocking.

PS : This is my very first pull request
Update contributors list
Improve mobile version of Green theme
Green theme : improvements

I hope I'm requesting this pull correctly
Merge pull-request for the green theme
Merge branch 'master' of git://github.com/jamesscottbrown/miniflux into popup
Improve help popup (pull-request)