Compile: remove "register" keyword in MD5Transform

"register" is usually ignored by the compiler and
is deprecated in C++11

[-Werror,-Wdeprecated-register]

Change-Id: I3a10f2128e4a4574b2cd3861bddbbd4ba6a3683f
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Add a benchmark test for QCryptographicHash

Some quick benchmarks against GNU coreutils 8.21 and OpenSSL 1.0.1e
(time in µs; time for coreutils and OpenSSL include the loading of the
executable):

             Qt                 Coreutils                OpenSSL
n    SHA-1 SHA-224 SHA-512  SHA-1 SHA-224 SHA-512  SHA-1 SHA-224 SHA-512
0      0      0       0      717    716     700     2532   2553    2522
64k   120    484     381     927   1074     966     2618   2782    2694
Diff  120    484     381     210    358     266       86    229     172

The numbers for Qt are pretty stable and vary very little; the numbers
for the other two vary quite a bit, since they involve launching and
executing separate processes. We can take the lesson that we're in the
same ballpark for SHA-1 and we should investigate whether our SHA2
implementation is sufficiently optimized.

Change-Id: Ib081d002ed57c4f43741eca45ff5cd13b97b6276
Reviewed-by: Richard J. Moore <rich@kde.org>