add optional passing of fw version

[maclobdell]

Resolves #26

[moshe-shahar]

Suggested change

	if fw_version is None:
	cmd = [MANIFEST_DEV_TOOL, 'create-v1', '-u', firmware_url, '-o', output]
	else:
	cmd = [MANIFEST_DEV_TOOL, 'create-v1', '-u', firmware_url, '-o', output, '--fw-version', fw_version]
	cmd = [MANIFEST_DEV_TOOL, 'create-v1', '-u', firmware_url, '-o', output]

Better to add it below Line 92 to include v3 too:

if fw_version:
    cmd += [ '--fw-version', fw_version ]