* Allow AddRange to work with int64_t.
Fixes#516
Also, tweak how we manage per-test build needs, and create a standard
_gtest suffix for googletest to differentiate from non-googletest tests.
I also ran clang-format on the files that I changed (but not the
benchmark include or main src as they have too many clang-format
issues).
* Add benchmark_gtest to cmake
* Set(Items|Bytes)Processed now take int64_t
