Re-enabled debugging prints in poplib & documented the set_debuglevel()

method.
This closes SF patch #486079.

Doc/lib/libpoplib.tex

@@ -55,11 +55,19 @@ in lower-case; most return the response text sent by the server.
 An \class{POP3} instance has the following methods:
 
 
+\begin{methoddesc}{set_debuglevel}{level}
+Set the instance's debugging level.  This controls the amount of
+debugging output printed.  The default, \code{0}, produces no
+debugging output.  A value of \code{1} produces a moderate amount of
+debugging output, generally a single line per request.  A value of
+\code{2} or higher produces the maximum amount of debugging output,
+logging each line sent and received on the control connection.
+\end{methoddesc}
+
 \begin{methoddesc}{getwelcome}{}
 Returns the greeting string sent by the POP3 server.
 \end{methoddesc}
 
-
 \begin{methoddesc}{user}{username}
 Send user command, response should indicate that a password is required.
 \end{methoddesc}