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

Previously the benchmark_test program executed the benchmark tests to make sure
the API was working but was not checking the number of tests that were
completed. If the regular expression matching breaks, zero tests could be ran.
Similarly, if one of the APIs breaks and doesn't run the correct amount of tests
then `make test` will catch this.

CONCAT was added quite late in the CMake release cycles so lets use `set()`
instead which has been around _forever_...

Verify gtest with URL_MD5

Verify the download of gtest