Merge #708: Constant-time behaviour test using valgrind memtest.

Merge #708: Constant-time behaviour test using valgrind memtest.

08fb6c49261f8aefaaa8ea2ca6d84a53e037e812 Run valgrind_ctime_test in travis (Jonas Nick)
3d2302257f19533932cd53547e9745b6283a907d Constant-time behaviour test using valgrind memtest. (Gregory Maxwell)

Pull request description:

  Valgrind does bit-level tracking of the "uninitialized" status of memory,
   property tracks memory which is tainted by any uninitialized memory, and
   warns if any branch or array access depends on an uninitialized bit.

  That is exactly the verification we need on secret data to test for
   constant-time behaviour. All we need to do is tell valgrind our
   secret key is actually uninitialized memory.

  This adds a valgrind_ctime_test which is compiled if valgrind is installed:

  Run it with libtool --mode=execute:
  $ libtool --mode=execute valgrind ./valgrind_ctime_test

ACKs for top commit:
  sipa:
    ACK 08fb6c49261f8aefaaa8ea2ca6d84a53e037e812
  real-or-random:
    ACK 08fb6c49261f8aefaaa8ea2ca6d84a53e037e812
  jonasnick:
    ACK 08fb6c49261f8aefaaa8ea2ca6d84a53e037e812

Tree-SHA512: d2eb829fb09f43ad1af70898e0eb9cf3f002c6bc418eca9e3e01a9c2c6e87c092aed23d6b0f311ddccbce1cce5f8ef39162cf9b2e68b83d160bc3d249e881493