Be consistent about which CSPRNG we use (For #19)

 * Any time random data is used, OpenSSL's RAND_bytes() is now called instead
   of evutil_secure_rng_get_bytes().
 * Use the numerics library's uniform_int_distribution instead of duplicating
   the stupid gen_padlen() routine with slightly different parameters.
 * The obfs2 pad length generation routine no longer asserts once in every
   8192 invocations.