Simple benchmarking framework

Benchmarking framework, loosely based on google's micro-benchmarking
library (https://github.com/google/benchmark)

Wny not use the Google Benchmark framework? Because adding Even More Dependencies
isn't worth it. If we get a dozen or three benchmarks and need nanosecond-accurate
timings of threaded code then switching to the full-blown Google Benchmark library
should be considered.

The benchmark framework is hard-coded to run each benchmark for one wall-clock second,
and then spits out .csv-format timing information to stdout. It is left as an
exercise for later (or maybe never) to add command-line arguments to specify which
benchmark(s) to run, how long to run them for, how to format results, etc etc etc.
Again, see the Google Benchmark framework for where that might end up.

See src/bench/MilliSleep.cpp for a sanity-test benchmark that just benchmarks
'sleep 100 milliseconds.'

To compile and run benchmarks:
  cd src; make bench

Sample output:

Benchmark,count,min,max,average
Sleep100ms,10,0.101854,0.105059,0.103881

1 files changed, 16 insertions, 0 deletions

diff --git a/src/bench/MilliSleep.cpp b/src/bench/MilliSleep.cpp
new file mode 100644
index 0000000000..991397a234
--- /dev/null
+++ b/src/bench/MilliSleep.cpp
@@ -0,0 +1,16 @@
+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "bench.h"
+#include "main.h"
+#include "utiltime.h"
+
+static void Sleep100ms(benchmark::State& state)
+{
+    while (state.KeepRunning()) {
+        MilliSleep(100);
+    }
+}
+
+BENCHMARK(Sleep100ms);