Create Minimal_Theme_Preparation

[jjgrainger]

Adds the Minimal_Theme_Preparation class and wp-empty-theme theme.

• Addresses acceptance criteria in Create Use_Minimal_Theme_Preparation  #39
• While writing test the get_theme_root() method was failing. Looking into this the get_raw_theme_root() function calls the get_theme_roots() function which uses the theme_roots transient to search for a valid theme root based on the theme slug. Tests were failing because the theme_roots transient wasn't being updated when registering the new theme directory with register_theme_directory() in the prepare method. I've included a search_theme_directories( true ); function call after registering the theme directory in the prepare method in order to force update this transient.

Closes #39

[felixarntz]

@jjgrainger Mostly looks good. Only one thing to fix in the code I think, but also we should add unit tests.

[felixarntz]

@jjgrainger Left some feedback on the test class.

[felixarntz]

@jjgrainger Almost good to go now, left 3 small final comments.

[mukeshpanchal27]

Thanks @jjgrainger, Left few final feedback and questions.

[mukeshpanchal27]

Thanks @jjgrainger, Only one out standing review #56 (comment)

[felixarntz]

Thanks @jjgrainger!