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

Fix #72 by avoiding 64-to-32-bit shortenings