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

Build triggered.

Build Started: https://ia-builds.doit.wisc.edu:8443/job/uw-php-security-master-pull-requests/4/

Oops, will need updates to the README too.

Build finished. Tests FAILED. Build results available at: https://ia-builds.doit.wisc.edu:8443/job/uw-php-security-master-pull-requests/4/

Added 1 commit:

• 21f95fe3 - Update README

Build triggered.

Build Started: https://ia-builds.doit.wisc.edu:8443/job/uw-php-security-master-pull-requests/5/

Build finished. Tests FAILED. Build results available at: https://ia-builds.doit.wisc.edu:8443/job/uw-php-security-master-pull-requests/5/

Looks like test reports are causing the build failure:

[xUnit] [ERROR] - Test reports were found but not all of them are new. Did all the tests run?
  * E:\Jenkins\workspace\uw-php-security-master-pull-requests\reports\phpunit.xml is 5 days 0 hr old

[xUnit] [INFO] - Failing BUILD.

@andrew-summers I just merged !1 (merged), which is necessary for the Jenkins job to complete successfully. Please pull from upstream master and rebase onto this branch.

Added 6 commits:

• 21f95fe3...fdbef3b2 - 4 commits from branch adi-ia:master
• 06e1cdbb - Refactor uw-php-security to use 'Provider' classes
• a54f5cf2 - Update README

@ahoffmann thanks, and done.

Build triggered.

Build Started: https://ia-builds.doit.wisc.edu:8443/job/uw-php-security-master-pull-requests/6/

Build finished. Tests PASSED. Build results available at: https://ia-builds.doit.wisc.edu:8443/job/uw-php-security-master-pull-requests/6/

This is overall a much cleaner implementation, nice work. Let me know if you have any questions regarding my comments.