Revert references to classes in global namespace

Some references to classes were incorrectly assumed to be a reference
to self, whereas they were references to classes in global namespace
outside of this project.

FIX #37

src/PhpSpreadsheet/CachedObjectStorage/Memcache.php

@@ -253,7 +253,7 @@ class Memcache extends CacheBase implements ICache
             $this->cachePrefix = substr(md5($baseUnique), 0, 8) . '.';
 
             //    Set a new Memcache object and connect to the Memcache server
-            $this->memcache = new self();
+            $this->memcache = new \Memcache();
             if (!$this->memcache->addServer($memcacheServer, $memcachePort, false, 50, 5, 5, true, [$this, 'failureCallback'])) {
                 throw new \PhpOffice\PhpSpreadsheet\Exception("Could not connect to MemCache server at {$memcacheServer}:{$memcachePort}");
             }

src/PhpSpreadsheet/Writer/PDF/DomPDF.php

@@ -88,7 +88,7 @@ class DomPDF extends Core implements \PhpOffice\PhpSpreadsheet\Writer\IWriter
         }
 
         //  Create PDF
-        $pdf = new self();
+        $pdf = new \DOMPDF();
         $pdf->set_paper(strtolower($paperSize), $orientation);
 
         $pdf->load_html(

src/PhpSpreadsheet/Writer/PDF/MPDF.php

@@ -89,7 +89,7 @@ class MPDF extends Core implements \PhpOffice\PhpSpreadsheet\Writer\IWriter
         }
 
         //  Create PDF
-        $pdf = new self();
+        $pdf = new \Mpdf\Mpdf();
         $ortmp = $orientation;
         $pdf->_setPageSize(strtoupper($paperSize), $ortmp);
         $pdf->DefOrientation = $orientation;

src/PhpSpreadsheet/Writer/PDF/TcPDF.php

@@ -87,7 +87,7 @@ class TcPDF extends Core implements \PhpOffice\PhpSpreadsheet\Writer\IWriter
         }
 
         //  Create PDF
-        $pdf = new self($orientation, 'pt', $paperSize);
+        $pdf = new \TCPDF($orientation, 'pt', $paperSize);
         $pdf->setFontSubsetting(false);
         //    Set margins, converting inches to points (using 72 dpi)
         $pdf->SetMargins($printMargins->getLeft() * 72, $printMargins->getTop() * 72, $printMargins->getRight() * 72);