Replace cppunit with the googletest framework

The GTest framework has a more active support base, and natively
supports CMake. Introduce it as an external dependency (using CMake's
ExternalProject_Add), replacing cppunit and porting the associated unit
tests.
Merge pull request #372 from jdduke/gtest_pr_final

Replace cppunit with the googletest framework
Add tests for fast_atof

The custom implementation for fast_atof currently lacks test coverage,
making it susceptiple to regressions with future changes. Add some
basic test coverage, in addition to a few documented cases that are
known to fail.
Merge pull request #374 from jdduke/test_atof

Add tests for fast_atof
Remove std::isnan and std::isinf usage from utFastAtofTest

These functions were added in C++11, and should not be used nakedly
in the current code base. Replace them with suitable C++03 constructs.
Merge pull request #376 from jdduke/make_fast_atof_test_cpp03

Remove std::isnan and std::isinf usage from utFastAtof test
Merge remote-tracking branch 'upstream/master' into trunk
Merge remote-tracking branch 'upstream/master' into trunk