fix __init__.py for doing 'make html' on WindowsOS

I was trying to write docs on Windows 7 (x64). 
Under Windows 7 with sphinx , I need to fix "docs\_ext\aws\__init__.py" for doing "make html". So I commited two fixed line on the code.

1. Use os.path insted of "/", where only I need
2. Exchange " and ' in __load_php()'s sh literal.
    Windows "php -r" command can be passed "(doble-quote) sentence. (On my thinking, Linux "php -r' support single&double-quote)


Please check it , if something on my request, Please let's me know.

Thanks for great product.