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

Can provide speed improvements

This improves development iterations when working in debug mode as you
don't have to fix up warnings to get the code to compile. Once a feature
is complete you can then run a release build and fix up all the warnings.

This checks the flag on the compiler AND linker which allows it to work
with flags such as -flto and -fsanitize=address

Add ability to use benchmark fixtures

support it.

simplify thread linking logic.

they get the appropriate include directories and link libraries automatically.

Fix typos: rangeY -> range_y()

Remove BENCHMARK_ENABLE_SHARED and prefer BUILD_SHARED_LIBS