Add a command to access the system timer

Two sub-commands... start and get.
 * start sets the reference.
 * get prints out the time since the last start (in "<sec>.<msec>" format).
If get is called without start, returns time since boot.
Simple way to benchmark an operation: "timer start;<commands-to-measure>;timer get"

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>