fix randombytes_{random,uniform}

* these functions return `int` and, therefore, do not have `.raw`
  attribute
* add a positive test cases for each
Merge pull request #26 from sa2ajj/fix-random-and-uniform

fix randombytes_{random,uniform}
add a couple more tests for randombytes
Merge pull request #27 from sa2ajj/more-tests-for-random

add a couple more tests for randombytes