Replaced procfs dependent functions with native calls on OS X. #87

I started replacing functions in linux_main.cpp which rely on procfs with OS X native calls and separating the two by ```#ifdefs```

In the end it looked really confusing, so I decided to move all the common functions into posix_main.cpp and separate platform specific ones into platform_linux.cpp and platform_osx.mm (contains objective-c++ code). It's not finished, it's not tested on linux and it breaks clang build (gcc compilation works) with this error: 

```error: Objective-C 1 was disabled in PCH file but is currently enabled```

I wasn't sure if you were going to like this solution, so I submitted this pull request first before working on it further.