Resolve benchmark cleanup race condition in issue #20.

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.
Statically initialize benchmark_mutex and extend its lifetime.

RunSpecifiedBenchmarks destroys benchmark_mutex before its last usage,
typically in RemoveBenchmark during cleanup of the BenchmarkFamilies singleton.
Resolve race on approx_time_ in FastClock.

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.
Merge pull request #21 from ckennelly/issue20

Resolve benchmark cleanup race condition in issue #20.
Merge pull request #22 from ckennelly/mutex_initialization

Statically initialize benchmark_mutex and extend its lifetime.
Merge pull request #23 from ckennelly/clock_initialization

Resolve race on approx_time_ in FastClock.