benchmark

Allow AddRange to work with int64_t. (#548)

* 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