Avoid default source of randomness.

The default source of randomness is no longer used when generating
tokens, secrets, and other values that should be cryptographically
secure. Each `random.<method>()` uses the same source of randomness,
which can be re-seeded at any point in time by other code executing in
the same process. Instead, a new CSRNG
(http://docs.python.org/2/library/random.html#random.SystemRandom) is
used each time.
Add tests for the helper functions.
Bump version.