Add a simplesession option

simplesession represents a stripped down session.

For now it doesn't support many bells and whistles, but it
should suffice for one-off command usage.

Caller has to opt-in to the experimental simplesession by
calling select_simplesession() method in command:
import pyghmi.ipmi.command
pyghmi.ipmi.command.select_simplesession()

Change-Id: I3884762621306d4a895f4b56d173fc80526a095c

pyghmi/ipmi/command.py

@@ -84,6 +84,11 @@ power_states = {
 }
 
 
+def select_simplesession():
+    global session
+    import pyghmi.ipmi.private.simplesession as session
+
+
 def _mask_to_cidr(mask):
     maskn = struct.unpack_from('>I', mask)[0]
     cidr = 32