Minor changes

These are some changes I made while reading through the code to better understand how the library works:

* added the close() call to the logout method -- this is recommended according to imaplib documentation
* moved creation of self.server to __init__ in Imbox so that the Imbox object is never in an incomplete state
* added folders() method to Imbox interface. It seems like it should be on the same object that is used to query the messages.