Fixed MacOS package naming and icons

This also changes the app name for the .deb package as well.

NOTE:

macOS will complain that the app is from an unidentified developer when you try to run it. To get past this, open the Applications folder and "right-click -> Open" the GCal app. The dialog that displays should give the ability to open the app, and future opens will not be an issue.

On macOS you may also want to open the System Preferences / Notifications (cmd + space, type Notifications) and allow notifications for GCal. Otherwise, the first time a calendar event tries to notify you, you'll just get a popup about allowing notifications instead. Subsequent notifications will work.

Adds initial osx build support

Known issues:

• not using any app signing, so likely will complain about being
  from an unidentified developer depending on system preferences
• app icon is the default Electron icon at present, I'll look
  into that at some point

First release, Linux .deb packaging only

Add build/packaging for linux .deb

Package doesn't have a nice icon at install time
but looks fine in the task manager/task bar