Add snap support

[undef314]

Hi! I put together this PR to add a snap package. I've tested it on Ubuntu 16.04, but it should work just as well on Ubuntu 17.10, Ubuntu 14.04, Linux Mint, Manjaro, Debian, OpenSUSE, Solus, etc.

If you merge and npm run build it will create dist/firebase-admin_1.0.1_amd64.snap.

Copy this to a Linux system, enable snap support, then run:
sudo snap install firebase-admin_1.0.1_amd64.snap --dangerous

Run with firebase-admin or find it in the launcher.

If you create a developer account and push this to the Snap Store, it can be discovered and installed through GNOME Software and https://snapcraft.io/discover. To create the developer account, sign up here, then register the "firebase-admin" name.

You'll need the snapcraft command to push the snap file to the store.

If you're on a Mac, you can brew install snapcraft
If you're on Linux, it's sudo snap install --classic snapcraft
Then you can push Ling out with:
snapcraft push firebase-admin_1.0.1_amd64.snap --release stable

[undef314]

While building the snap if you get an error that 'icns2png' exec was not found in $PATH.
Then install icnsutils on your machine.
For Ubuntu : sudo apt-get install icnsutils

[ohtangza]

Is it a dead project?

[hb-webdev]

Sigh. This looked like a nice app. I can't even log in with my private key json file so can't use the app at all

[kristianalnaes]

@hb-webdev I'm not the maintainer, but testing the Windows version of the application works fine.

Please note that the Google Console outputs incorrect URL with an extra "firebase-" prepended to Database URL in the JSON credential file.