Added synopsis for Linux

changed name in package.json so that it's unique (especially important on Linux to avoid packages clashes)

Linux packaging 

Fix electron-builder failing on linux .deb package

Add 512x512 icons to fix electron-build for linux

Update electron-builder.json

Add icon files with correct dimensions to fix electron-builder fail for linux deb package. Requires minimum size of 512x512. (see https://www.electron.build/icons)

Provide correct path to build icons. The path must be a directory and not pointed to a filename.

build caches and portable package names

* added install stage in appveyor

* added install stage in appveyor2