Make Benchmark a single header library (but not header-only) (#407)

* Make Benchmark a single header library (but not header-only)

This patch refactors benchmark into a single header, to allow
for slightly easier usage.

The initial reason for the header split was to keep C++ library
components from being included by benchmark_api.h, making that
part of the library STL agnostic. However this has since changed
and there seems to be little reason to separate the reporters from
the rest of the library.

* Fix internal_macros.h

* Remove more references to macros.h