raspivid: Shutter speed option changes to accept rational number

Added shutter_speed_from_string() function to parse the string passed to the shutter speed command line option and if it contains a slash (/), use the MMAL rational number functions to convert it to microseconds.  If the string does not contain a /, it is assumed to be microseconds.
In short, values like 1/60 can be used to set shutter speed as well as microsecond integer value (ie 15000).
Forum thread: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=43&t=59299