handle lower encoding, add quoteprintable, fix attachment handling, f…

[ArkaGPL]

• Lower transfert-encoding as proposed by use case-insensitive string to compare the encoding #20
• Fix attachment handling in multipart/mixed as proposed by Find Attachments, dont matter the content-type msg #25
• Fix transfert-encoding support for html and plain which is sometimes base64, quotedprintable, 8bits, binary as proposed by Decode text/plain and text/html body with encoding #24
• Add quotedprintable transfert-encoding support