Add attachment support for the sending API, now only support text/plain.
Now support fetching the 'html' text of 'alternative' type through the retrieve API, move the retrieve relative functions to the retrieve_util module.
Add MIME type for attachments in the send API.
Set  []  as the default value of #mail.attachments.
Make the send API support 'text/html' type.
Use 'attachment' as the default filename when there is no 'filename' info of attachment of the email, instead of throwing an exception.
Add an empty file to keep ./ebin in track.
Add README
Don't call iconv when there is a decode error, use the original encoding type.
Add 'LOGIN' type authentication for smptc.
Do not transcode when there is an error in iconv(like elisq). Add a function mail_client:list_size/1 to return only the inbox messages' size.
Add 'Bcc' for the retrieve and send API.
Added ignore file.
Added mail_client:pop_capabilities/1 to check pop3 server if rfc2449 is supported.