Generate AppImage package for GNU/Linux x86_64

Fixes: #323
Signed-off-by: Juan Cruz Viotti <jviottidc@gmail.com>

Etcher.desktop

@@ -0,0 +1,6 @@
+[Desktop Entry]
+Name=Etcher
+Exec=etcher
+Description=Burn images to SD cards & USB drives, safe & easy.
+Icon=icon
+Type=Application

Makefile

@@ -203,6 +203,16 @@ etcher-release/installers/Etcher-linux-x86.tar.gz: etcher-release/Etcher-linux-x
 	mkdir -p $(dir $@)
 	tar -zcf $@ $<
 
+etcher-release/installers/Etcher-linux-x64.AppImage: etcher-release/Etcher-linux-x86
+	mkdir -p $(dir $<)Etcher-linux-x64.AppDir/usr/bin
+	cp ./scripts/AppRun $(dir $<)Etcher-linux-x64.AppDir
+	chmod a+x $(dir $<)Etcher-linux-x64.AppDir/AppRun
+	cp ./Etcher.desktop $(dir $<)Etcher-linux-x64.AppDir
+	cp ./assets/icon.png $(dir $<)Etcher-linux-x64.AppDir
+	cp -rf $</* $(dir $<)Etcher-linux-x64.AppDir/usr/bin
+	mkdir -p $(dir $@)
+	./scripts/AppImageAssistant $(dir $<)Etcher-linux-x64.AppDir $@
+
 etcher-release/installers/Etcher-win32-x64.exe: etcher-release/Etcher-win32-x64 package.json
 	$(ELECTRON_BUILDER) $< \
 		--platform=win \
@@ -225,7 +235,7 @@ package-win32: etcher-release/Etcher-win32-x86 etcher-release/Etcher-win32-x64
 package-all: package-osx package-linux package-win32
 
 installer-osx: etcher-release/installers/Etcher-darwin-x64.dmg
-installer-linux: etcher-release/installers/Etcher-linux-x64.tar.gz etcher-release/installers/Etcher-linux-x86.tar.gz
+installer-linux: etcher-release/installers/Etcher-linux-x64.tar.gz etcher-release/installers/Etcher-linux-x86.tar.gz etcher-release/installers/Etcher-linux-x64.AppImage
 installer-win32: etcher-release/installers/Etcher-win32-x64.exe etcher-release/installers/Etcher-win32-x86.exe
 installer-all: installer-osx installer-linux installer-win32