Add script changing NTP server address.
Fixes after Jimmy Kaplowitz review:

 * use sed with -i option instead of grep to avoid creation of temp file
 * add comment to ntp.conf file explaining configuration change
 * change comments explaining that NTP server is located on host machine
Remove existing lines with server addresses instead of commenting them.
Do not force sed to create backup copy.
Merge pull request #2 from rybaktomasz/google-master

Add script changing NTP server address