Support for new hierarchical structure in documentation for appium

-Support for restructuring of appium documentation to provide support to hierarchical
-Supports [This directory sturcture] (https://github.com/moizjv/appium/tree/documentation/docs/en) , it includes having directories in docs and also changing headings from `h1` to `h2` and `h2` to `h3` to accommodate new folder name to have heading `h1` and support 3 level hierarchy .
-Tested using 
     
    `rake md[../../appium/docs/en]` 
    `rake build` 
    `bundle exec middleman server`
-New yaml support for labeling folders, ignore list and ordering  
-Will look like this
![screen shot 2014-07-02 at 5 55 44 pm](https://cloud.githubusercontent.com/assets/1308607/3464442/0e37dbfe-024d-11e4-9697-b1b39ca47c7e.png)

This should be after `validate_dir`