Two bug fixes in KeepRunning

Resolve race on approx_time_ in FastClock.

Statically initialize benchmark_mutex and extend its lifetime.

Resolve benchmark cleanup race condition in issue #20.

InitType should hold a lock before storing to approx_time_, which is later
read by the background worker.  When the worker is actively running (i.e., not
blocked on bg_cond_) it holds bg_mutex_.

InitType is called during benchmark setup only, so any contention induced for
the mutex should not have performance/accuracy consequences.

RunSpecifiedBenchmarks destroys benchmark_mutex before its last usage,
typically in RemoveBenchmark during cleanup of the BenchmarkFamilies singleton.

The multithreaded API for benchmarks provides that teardown can happen in
thread 0.  For this to be safe, all other threads executing the benchmark
function need to have exited.  Otherwise, thread 0 may begin to teardown shared
resources before the other threads have stopped using these resources as they
are in their last loop of while (KeepRunning()) { ... }.

This change creates a single exit point for KeepRunning() to return false.
When running a multithreaded benchmark, thread 0 blocks on KeepRunning() until
all other threads have exited.  This approach allows for there to be no change
to the user-facing API exemplified in the BM_MultiThreaded example.

Remove URL_HASH and TLS_VERIFY from CMake configuration.

Per the CMake 2.8.0 documentation, these options did not exist for
ExternalProject_Add.  These options were added in CMake 2.8.10.

Resolve Memory Leaks #17

This shifts registration from a global vector to a singleton that manages
benchmark destruction during shutdown.

This adds a unit test to validate the wrapper without running the entirety of
benchmark_test.

Use a new container in each round of push_back test.

Otherwise containers keep growing and uses gigabytes of memory.

Unable to compile because of unused parameter

There is a unused parameter at benchmark:866 Benchmark::MeasurePeakHeapMemory .
The function is marked as TODO and the code that uses the parameter is commented out.
I just removed the parameter so it can compile again without editing the CMake compile flags.

Fix cycleclock.h for gcc/ARM.

Currently there are tests for ARMV3 and ARMV6 in cycleclock.h which are not
defined using gcc on ARM. Since there is also a cast to the unknown type
int64 I assume that the ARM code has not been tested. Therefore this patch
replaces the checks for ARMV3 and ARMV6 by checks for __ARM_ARCH. Also, the
cast to int64 is fixed by casting to int64_t.

Ensure families is not nullptr before using it

When there's no benchmarks, families becomes nullptr. So before touching
it, we need to ensure families is not nullptr.

Update README.md

Inspired from the one from https://github.com/google/go-github.