make 2nd, 3rd and 4th argument optional by supplying None as default

README.md

@@ -124,7 +124,7 @@ You can then test the converter using:
 Note that the package can also be used as a library as follows:
 
 	import img2pdf
-	pdf_bytes = img2pdf.convert(['test.jpg'], dpi=150, x=0, y=0)
+	pdf_bytes = img2pdf.convert(['test.jpg'])
 
 	file = open("name.pdf","wb")
 	file.write(pdf_bytes)

src/img2pdf.py

@@ -205,9 +205,9 @@ class pdfdoc(object):
         result += b"%%EOF\n"
         return result
 
-def convert(images, dpi, x, y, title=None, author=None, creator=None, producer=None,
-            creationdate=None, moddate=None, subject=None, keywords=None,
-            colorspace=None, verbose=False):
+def convert(images, dpi=None, x=None, y=None, title=None, author=None,
+            creator=None, producer=None, creationdate=None, moddate=None,
+            subject=None, keywords=None, colorspace=None, verbose=False):
 
     pdf = pdfdoc(3, title, author, creator, producer, creationdate,
                  moddate, subject, keywords)