Add custom_dotfiles_dir attribute.
Add custom_dotfiles_repo_prefix.
Update README.
SD-specific default attributes.
Update defaults.
Merge branch 'customize-dotfiles-dir-and-path-prefix' into sd-dotfiles-updates

Conflicts:
	attributes/default.rb
Update README.
Tweak README.
Merge pull request #1 from spanishdict/sd-dotfiles-updates

Sd dotfiles updates