Refactor uw-php-security to use 'Provider' classes

After a discussion about the project's design and requirements moving forward, it was decided to refactor the project to remove AttributeMapper classes and have Provider classes handle attribute mapping as well as user loading. This also adds a third provider, PreauthHTTPUserDetailsProvider, which can be used in environments where Shibboleth attributes are sent as HTTP headers.

Apologies for the big PR!

Please review: @ahoffmann

src/main/edu/wisc/doit/LocalUserDetailsAttributeMapper.php

-<?php
-
-namespace edu\wisc\doit;
-
-/**
- * Implementation of {@link UserDetailsAttributeMapper} for use in local development.
- */
-class LocalUserDetailsAttributeMapper implements UserDetailsAttributeMapper
-{
-
-    /**
-     * {@inheritdoc}
-     */
-    public function mapUser()
-    {
-        $jsonString = file_get_contents(__DIR__ . "/../../../resources/localuser.json");
-        if ($jsonString === false) {
-            return null;
-        }
-
-        // Return user attributes into a standard PHP array (true specifies array)
-        return json_decode($jsonString, true);
-    }
-
-}
\ No newline at end of file