Improved the definition of robot.random.uniform(), because it was ambiguous.

Now the semantics are as follows:

- random.uniform() always returns a floating-point number
  * random.uniform() works in the range [0,1)
  * random.uniform(max) works in the range [0,max)
  * random.uniform(min,max) works in the range [min,max)

- random.uniform_int() always returns an integer number
  * random.uniform_int(max) works in the range [0,max)
  * random.uniform_int(min,max) works in the range [min,max)

(thanks Andrea Roli for pointing this out)