Fix IMAP transport instantiation

This reverts the approach to IMAP transport instantiation back to how it
was done prior to 2a0117b6 which introduced the usesslcontext parameter.
This reduces code duplication and importantly make instantiation work
again because self.IMAP4 and self.IMAP4_SSL do not exist.

imbox/imap.py

@@ -11,22 +11,19 @@ class ImapTransport(object):
     def __init__(self, hostname, port=None, ssl=True, usesslcontext=True):
         self.hostname = hostname
         self.port = port
+        kwargs = {}
 
         if ssl:
-            if usesslcontext is True:
-                context = pythonssllib.create_default_context()
-            else:
-                context = None
-
+            self.transport = IMAP4_SSL
             if not self.port:
                 self.port = 993
-            self.server = self.IMAP4_SSL(self.hostname, self.port,
-                                         ssl_context=context)
+            kwargs["ssl_context"] = pythonssllib.create_default_context()
         else:
+            self.transport = IMAP4
             if not self.port:
                 self.port = 143
 
-        self.server = self.IMAP4(self.hostname, self.port)
+        self.server = self.transport(self.hostname, self.port, **kwargs)
         logger.debug("Created IMAP4 transport for {host}:{port}"
                      .format(host=self.hostname, port=self.port))