Add linux target to build task

[louisbourque]

Hello,
This pull request adds a linux build task to hopefully resolve #7.
I had to add the icons as separate PNGs due to this bug I encountered: electron-userland/electron-builder#239 (comment)

This pull request also fixes a bug where when closing Partyshare with the X button, the icon would continue to show, but clicking on it does nothing. I changed it so the ipfs daemon is stopped when the X button is pressed. Not sure if this was the intended functionality of the button,but I can change if necessary.

[bhstahl]

Great! Thanks for this PR @louisbourque! I'm almost ready with Release 1.1.0. Let's merge this with that release.

[louisbourque]

Awesome! Thanks for accepting my pull request. I'm happy I was able to contribute.

I'm not very familiar with how electron works. Does this mean you can now generate the Linux builds? When I was working on it, it didn't seem to build a Mac .dmg file. I can add the link to the gh-pages branch, but I'm not sure how the publish step works.

Also, just a heads up, the 'fix_asar' gave me errors (sed: can't read s|run(cmd, args|run(cmd.replace("app.asar", "app.asar.unpacked"), args|: No such file or directory), so I just manually changed the "node_modules/ipfsd-ctl/src/exec.js" to use const command = run(cmd.replace("app.asar", "app.asar.unpacked"), args, opts). Once #14 is fixed, this won't be a problem any more of course.