Do not use UNIX signals in Windows

This patch does not use the signals in Windows. just avoid these calls.

This patch is related to issue #37, issue 3.

offlineimap/init.py

@@ -26,6 +26,7 @@ import collections
 from optparse import OptionParser
 
 import offlineimap
+from offlineimap.utils.distro_utils import get_os_name
 import imaplib2 as imaplib
 
 # Ensure that `ui` gets loaded before `threadutil` in order to
@@ -457,6 +458,9 @@ class OfflineImap:
 
         try:
             self.num_sigterm = 0
+
+            # We cannot use signals in Windows
+            if get_os_name() != 'windows':
                 signal.signal(signal.SIGHUP, sig_handler)
                 signal.signal(signal.SIGUSR1, sig_handler)
                 signal.signal(signal.SIGUSR2, sig_handler)