This patch adopts a new internal structure for how timings are performed.
Currently every iteration of a benchmark checks to see if it has been running
for an appropriate amount of time. Checking the clock introduces noise into
the timings and this can cause inconsistent output from each benchmark.

Now every iteration of a benchmark only checks an iteration count to see if it
should stop running. The iteration count is determined before hand by testing
the benchmark on a series of increasing iteration counts until a suitable count
is found. This increases the amount of time it takes to run the actual benchmarks
but it also greatly increases the accuracy of the results.

This patch introduces some breaking changes. The notable breaking changes are:
1. Benchmarks run on multiple threads no generate a report per thread. Instead
   only a single report is generated.
2. ::benchmark::UseRealTime() was removed and replaced with State::UseRealTime().

Enable UseRealTime and fix documentation for SetLabel.

Fixes #89

UseRealTime was defined in the internal namespace by mistake.
Similarly, documentation suggested that benchmark::SetLabel should be
used to set a label, and a function was declared but not defined, while
actually the call should be 'state.SetLabel'.

Add filter test, remove re test, and googletest deps

Step three towards moving timer changes.

Step two towards merging timer changes.

Step one towards merging timer changes.

This patch does two things:

1. It overhalls the static initialization in Walltime to be simplier. It uses
   a static variable inside WallTime::Now() to initialize the timer.

2. Add a logging mechanism so that the -v flag actually has meaning and
   reimplement the CHECK macros to allow extra messages to be streamed in.

This patch cleans up our use of generic macros and also merges changes in the
build system.

It adds options -DBENCHMARK_ENABLE_TESTING and -DBENCHMARK_ENABLE_SHARED.

libstdc++'s std::regex has (or had) a bug in std::regex::operator=(...) that
caused undefined behaviour. Clang will detect this and compile the function so
that it crashes at runtime. This patch tried to detect that bug during
configuration.

Add _build to gitignore to enable out-of-source builds

Fix #77 build on freebsd 10

One more 64-bit to 32-bit conversion