Allow automatic Unit-e daemon download

[cmihai]

Describe the bug
The functionality to check the Unit-e GitHub repository for the daemon package and automatically download and unpack it is currently commented out, because there are no public daemon packages yet.
As a result, right now the user has to start the daemon manually before running the wallet. We should uncomment and test the download functionality before the testnet is live.

To Reproduce
Steps to reproduce the behavior:

1. Launch the unite-desktop Electron wallet
2. Notice that it's failing to connect to the daemon unless united is already running on the machine.

Expected behavior
The wallet automatically downloads the daemon package, extracts it in a directory under $HOME, and runs it.

[cornelius]

I suppose it's implied in the description but just to make it explicit: There needs to be a check that the downloaded package is a trusted build of the client and nobody has tampered with it or the way how it's downloaded.