Add more GMail folders

I really shouldn't need to add every single folder to the C++ code,
but I guess this will have to suffice for now...
Change the CA file

I have absolutely no idea what I'm supposed to stick here.  The old
file appears to have gone away, but the new one is probably
GMail-specific.
Add mhdir::folders() to list all folders

This lets me implement the "folders" command, which gives some basic
information about the folders in my system.
Add folder::message_count()

This counts the number of messages in the current folder.  While it's
probably possible to do this using SQL, I'm just iterating through
every message.
Add the "folders" command

This prints some basic information about the folders present on a
system.  It's not super useful, but I figured I needed something so I
could poke the code again.