Improve documentation

[fwolfst]

• fix a copy/paste error in Mail#last
• prior to conversation: fix #decode example in README. Here I am not 100% sure but calling #decode on a mail-object gives me Can not decode an entire message, try calling #decoded on the various fields and body or parts if it is a multipart message.. Although I'd find it pretty cool if #decode gives me whatever text/body-part there is.
• after conversation: Slightly improved message of NoMethodError in Message#decode. Agreed on further improvements should include choosing a more distinct Error class.