Check if hashes match after downloading Parity

[axelchalon]

Should prevent #119
The sha256 hash of the binary is provided in the json response that we get the Parity binary URL from. We might want to use streams for checking the hashes. Would be similar to:

shell/src/util/hashFetch/index.js

Lines 42 to 61 in c4f1516

	function checkHashMatch (expectedHash, path) {
	return new Promise((resolve, reject) => {
	const hasher = keccak256.create();
	const fileReadStream = fs.createReadStream(path);
	fileReadStream.on('end', () => {
	const actualHash = hasher.hex();
	if (actualHash !== expectedHash) {
	reject(`Hashes don't match: expected ${expectedHash}, got ${actualHash}`);
	} else {
	resolve();
	}
	});
	fileReadStream.on('data', (chunk) => {
	hasher.update(chunk);
	});
	});
	}

[axelchalon]

We already implement this check when downloading Parity in fether (and we'll soon check for interrupted downloads: openethereum/fether#173)

https://github.com/paritytech/fether/blob/cf01b3f713bfb85d04bc4c3e061420aca1ac22e3/packages/parity-electron/src/fetchParity.ts#L107-L117

This happens in the @parity/electron package of the fether repo. This package will soon be moved to paritytech/js-libs, so we can simply wait for this to happen and then use this library in Parity UI.