Added check for authorization header

:koala: :+1: 

This PR adds a check for the authorisation header to provide more meaningful logging. The method [`getHeader`](http://www.pata.cat/tools/zend-framework-1.10/doxygen/class_zend___controller___request___http.html#aab81e43916cdc2d7c29fd4beb9f52519) firstly tries to extract the headers from the [`$_SERVER`](http://php.net/manual/en/reserved.variables.server.php) array. If it can't find it there it tries the [`apache_request_headers`](http://www.php.net/manual/en/function.apache-request-headers.php) function, which was not available under FastCGI until PHP 5.4. This results in some servers failing to authenticate with the API.

/cc @jwswj @maximeprades @chnorton