Move API key authentication into a method

Eliminates duplicate code and standardizes functionalty within
class.API.inc.php. Per #420.
Include the search result Solr score

Per #225.
Eliminate a redundant Solarium autoloader registry

This is already established in index.php, per #426.
Retrieve GET variables from their proper location
Add a new route for the term suggestion API method

Per #438.
Establish an API method for autosuggesting terms

In order to provide live search suggestions, per #438.