Fix issue #141. Use size_t instead of int for the iteration count

Remove std::is_trivially_destructible assertion. Fix issue #134.

Updates Initialize() to work with an argv as 'char**'

Jleroy/vs2013 compatibility

Fix documentation comment in benchmark_api.h w.r.t CustomArguments

Fix doc sample typos in header and README

* iterations() is a method
* int64_t fix in a couple of places

Fix the compilation error against G++ on Mac OS X.

Using `0` as a null pointer is illegal when `-Wzero-as-null-pointer-constant`
is given to G++.  To avoid the warning `zero-as-null-pointer-constant`,
`nullptr` (C++11 keyword) instead of `0` is used in the `sysctl` invocation.

Fix issue #127

Fixing cmake issue with empty cmake_build_type

Coverage

This patch adds a `coverage` target that allows coverage statisitics to be
retrieved for the project. It requires that lcov and gcov is installed and
that the generator is unix makefiles but this can be improved upon in
future releases.

To make it work use the coverage build type:

```
cmake -DCMAKE_BUILD_TYPE=Coverage .
make coverage
```

This is useful to test the library at different standard levels. Currently
doesn't search for C++14 in 'auto' mode. Can enable in the future