MacOS DMG packaging

Update README.md [skip ci]

Travis is now running for Linux/Mac builds thus can remove the "(coming soon)".

Updating download links for windows, and  linux tarball, and rewording the notes for OS X installers

Change naming of package files

Easier reading/identifying of package/installation files based on common naming practice:

windows:       Breeze Wallet_v0.3.0_win64_setup.exe 
linux:             Breeze Wallet_v0.3.0-linux_amd64.deb
mac:               Breeze Wallet_v0.3.0_osx64.dmg

Fix mac/linux config and style cleanup

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)