Add function thematic_available_theme_layouts()

Specify an array of available theme layouts
Add filter `thematic_available_theme_layouts`
Use thematic_available_theme_layouts() for the body classes
Use thematic_available_theme_layouts() in options validation
Use thematic_available_theme_layouts() in Theme Customizer
Add unit tests for theme layout filters
Add filter `thematic_default_theme_layout` for default layouts
Add more unit tests for layout filters

Add test for trying to switch to nonexistent layout
Add test for changing the default layout
Add test for switching to nonexisting layout when default
  layout has been changed
Add utility function thematic_available_layout_slugs()
Move theme layout functions to helpers.php
Correct typo in return statement of thematic_available_layout_slugs()

In helpers.php. Remove parenthesis from return variable
Use helper function thematic_default_theme_layout() where applicable
Fix undefined layout variable in thematic_validate_opt()
Merge pull request #97 from ThematicTheme/layout-filters

add filter for the main layouts