Added a 'a is true/false withProbAtLeast n' combinator.

I sometimes wish to have something like this to write in my `if` statements:
```scala
scala> if (ApproximateBoolean(true,0.85) is true withProbAtLeast 0.9) println("yay") else println("nay")
yay
```
Thought, it might also be useful to have such a combinator.
```scala
scala> ApproximateBoolean(true, 0.8) is true withProbAtLeast 0.8
res0: Boolean = true

scala> ApproximateBoolean(true, 0.8) is true withProbAtLeast 0.75
res1: Boolean = false

scala> ApproximateBoolean(true, 0.8) is true withProbAtLeast 0.85
res2: Boolean = true

scala> ApproximateBoolean(true, 0.8) is false withProbAtLeast 0.2
res3: Boolean = true

scala> ApproximateBoolean(true, 0.8) is false withProbAtLeast 0.25
res4: Boolean = false

scala> ApproximateBoolean(true, 0.8) is false withProbAtLeast 0.15
res5: Boolean = true

scala> ApproximateBoolean(false, 0.5) is false withProbAtLeast 0.5
res6: Boolean = true

scala> ApproximateBoolean(false, 0.5) is true withProbAtLeast 0.5
res7: Boolean = true

scala> ApproximateBoolean(false, 0.5) is true withProbAtLeast 0.45
res8: Boolean = true

scala> ApproximateBoolean(false, 0.5) is false withProbAtLeast 0.55
res9: Boolean = true
```