Add number of threads to State.

Having access to the thread count from within a benchmark is useful,
for when one wants to distribute a workload dynamically among the
benchmarks running in parallel e.g when using ThreadRange() or
ThreadPerCpu().

Add section on optimisations to README

Add aarch64 support to src/cycleclock.h.

Fix ARM /proc/cpuinfo parsing.

Support MSVC on appveyor

Also fix related warning message typos.

From google3's base/cycleclock.

Update README to mention UseRealTime for wallclock time measurements.

Also adding a use case in the API header.

Fixes #170

Fixes #165: CustomArguments ret type in README

* adds myself to AUTHORS/CONTRIBUTORS

Fixes #159: Add inclusion of cstdio to files using stdout, printf and etc.

add static_cast to get rid of warnings for intel compiler on linux platform

Fix get_git_version to use GIT_EXECUTABLE.

get_git_version CMake function uses 'git' command directly, instead of
GIT_EXECUTABLE variable. This causes CMake errors while generating
project files in environments, where 'git' command is not present
in PATH.

Fixes #144  CMake fails without Git 

Android fix

The sane thing here would be to use std::stoul but this is not available in the android-ndk...

- Added missing header to test/benchmark_test.cc
- Changed std::stoul to std::atol in test/filter_test.cc because of a
  limitation in the android-ndk (http://stackoverflow.com/questions/17950814/how-to-use-stdstoul-and-stdstoull-in-android)

sys/sysctl.h is not available in the android-ndk.

Fixes compilation issue on Macs

ba141ac0 renamed OS_MACOSX -> BENCHMARK_OS_MACOSX,
except for an include guard in src/cycleclock.h

Visual studio warnings fix