Windows support hacks

I really wanted to use knife-block on windows. So i came up with some kinda ugly hacks to make it work by redefining some File class method to shell out to the mklink cmd.exe function. 

I also changed ```def get_config_file``` to just return any found config file and moved the Dirname logic to the callee to match the function name and similar knife function. I also provided a default config directory for cases when knife.rb does not exist. 

I did not test this on OSX after my changes. please check on OSX before publishing.

Mind bumping the version and publishing a new gem?