Avoid errors when uploading large files (#94)

Reading/writing a large file all at once can cause a MemoryError or
an OverflowError. Using `shutil.copyfileobj` seems to solve the issue.

shotgun_api3/shotgun.py

@@ -4040,7 +4040,9 @@ def encode(self, params, files, boundary=None, buffer=None):
             buffer.write('Content-Type: %s\r\n' % content_type)
             buffer.write('Content-Length: %s\r\n' % file_size)
             fd.seek(0)
-            buffer.write('\r\n%s\r\n' % fd.read())
+            buffer.write('\r\n')
+            shutil.copyfileobj(fd, buffer)
+            buffer.write('\r\n')
         buffer.write('--%s--\r\n\r\n' % boundary)
         buffer = buffer.getvalue()
         return boundary, buffer