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

Using Travis CI.

Use #ifdef to protect calling Factorial in benchmark_test.cc

Added Travis build status badge.

Remove CHECK_* from public API.

Fixes #65

CHECK_* are now private and used internally in the library. The test
uses have been replaced with asserts.

Fixes #62.

added googletest dependency to tests (allows compilation with -j)

Turn the global mutex into instance variables

Fix #50 by using nullptr and adding stricter warning.

Fix #54 by correcting reference to CXX flags for variants.

Move pthread dependency to test CMakeLists.txt

To fix #52

Make sure all benchmark tests run