add random/randBool. also allow overriding the mockRandom values.
use `>= 0.5` instead of `> 0.5` on randBool

`random()` lower-bound is zero inclusive and upper-bound is one exclusive; and
`0.5` usually rounds up.
reuse `randBool` on `randBit` and `randSign`
Merge pull request #157 from mout/randBool

add random/randBool and reuse it internally