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, 5 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 67e848be39..45346df954 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -61,6 +61,7 @@ endif
 
 bin_PROGRAMS =
 TESTS =
+BENCHMARKS =
 
 if BUILD_BITCOIND
   bin_PROGRAMS += bitcoind
@@ -445,6 +446,10 @@ if ENABLE_TESTS
 include Makefile.test.include
 endif
 
+if ENABLE_BENCH
+include Makefile.bench.include
+endif
+
 if ENABLE_QT
 include Makefile.qt.include
 endif