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

Implemented git versioning

This patch automatically versions the shared libraries from any annotated `git`
tags:

```
git tag -a v1.0.0
```

It expects semver version tags such as `v1.0.0`. It would be trivial to support
`1.0.0` but looking around it seems that most C/C++ projects follow `vX.X.X`
rather that `X.X.X` like a lot of `Node.js` stuff.

This determines that the if the project has had a certain amount of commits
since the last tag and also if the project is _dirty_ (has modified files), but
does __nothing__ with that information. In the future a more robust release
could be implemented in the script.

This is pretty brittle and has little in the way of configuration. Ideally we
should use `find_program` to work out where `git` is so that users can configure
it. This implementation assumes that `git` will be available in `PATH`

Outputs the following on the command line:

```
-- git Version: v[MAJOR].[MINOR].[PATCH]-[COMMITS_SINCE_TAG]-[SHA1](-dirty)?
-- Version: [MAJOR].[MINOR].[PATCH]
```

Allow shared libraries with BUILD_SHARED_LIBS

Add Lei Xu to AUTHORS and CONTRIBUTORS

fix examples to use SetBytesProcessed

- use State::SetBytesProcessed not SetBenchmarkBytesProcessed

fix building on ubuntu