add built-in functions for cond

[tengattack]

Hi,

This time I add some built-in functions for cond module:

• empty() Checks if the argument is nil
• strlen() Returns the string argument's length
• rand() Returns a pseudo-random number in [0.0,1.0) as a float64

This may help us to achieve more kinds of jobs. For example, we can do output sample here by using rand() > 0.5

BTW, I update the README for previous typo in #40.