Add function to check on OS

[bHimes]

eg.

#!/usr/bin/env bash

if [ "$(uname)" == "Darwin" ]; then
    # Do something under Mac OS X platform        
elif [ "$(expr substr $(uname -s) 1 5)" == "Linux" ]; then
    # Do something under GNU/Linux platform
elif [ "$(expr substr $(uname -s) 1 10)" == "MINGW32_NT" ]; then
    # Do something under 32 bits Windows NT platform
elif [ "$(expr substr $(uname -s) 1 10)" == "MINGW64_NT" ]; then
    # Do something under 64 bits Windows NT platform
fi