Refactor structure of plugin
- Main logic begins in clef-require.php
- wpclef.php maintains constant indicating whether Clef is the base plugin
- Classes no longer inherit from ClefBase, which was becoming a repository for random functions
- Classes now implemented as singletons rather than static classes
- Plugin-setup related functions now in separate class, ClefSetup
- Site options/settings access now encapsulated in separate class, ClefInternalSettings
- Admin menu only rendered when Clef is base plugin
- Activation/uninstall hooks only registered when Clef is base plugin
- Several unused functions have been removed

(Sorry for the huge commit.)
fix version number for 'npm install'
ensure plugin hooks are registered at right time
allow clef settings path to be overridden by filter
add hook for rendering settings
don't create class if it already exists
Merge branch 'two-point-oh' into general-refactoring

Conflicts:
	includes/class.clef-login.php
fix static call to setting
remove extraneous echo
Merge pull request #72 from clef/general-refactoring

General refactoring