Use absolute import everywhere

This prevents Python 2.7 from attempting to import system modules
like sys and ctypes from the 'serial' directory.
diff --git a/serial/urlhandler/protocol_alt.py b/serial/urlhandler/protocol_alt.py
index c14a87e..2e666ca 100644
--- a/serial/urlhandler/protocol_alt.py
+++ b/serial/urlhandler/protocol_alt.py
@@ -16,6 +16,8 @@
 #   use poll based implementation on Posix (Linux):
 #   python -m serial.tools.miniterm alt:///dev/ttyUSB0?class=PosixPollSerial
 
+from __future__ import absolute_import
+
 try:
     import urlparse
 except ImportError: