Update some WordPress API calls and some PHP 7 support

[skoranda]

Added multi-site support for switch to blog functionality and updated calls to use the newer WordPress user object.

Replace deprecated get_usermetadata call with get_user_meta.

Check for index 'redirect_to' for $_REQUEST before calling shibboleth_session_initiator_url to avoid PHP notice.

Updated deprecated call update_usermeta to update_user_metada.

Replaced a call to the split() function with explode() since split is deprecated in PHP 5.3 and is no longer available in PHP 7.

[jrchamp]

Looks good. I have a slight preference for !empty($var) over isset($var) && $var, but the effect is the same.

Note: This pull request would close #9 and close #15

[michaelryanmcneill]

Hello, thank you for submitting this patch. I released version 1.8 today to resolve this and other issues and included a shoutout for your patch. I did not include the changes for "multi-site support for switch to blog functionality", as I was hoping you could provide further clarification on the purpose and use case that this is supporting. I am the new maintainer of the plugin and all further work on the plugin will be done in a new GitHub repository. If you believe that switch_to_blog change should still be included, can you please open up an issue/PR in the new repository with details on the use case? Thanks!