added support for RFC 2231. added support for non-ascii encoded filenames. refactored Mime4jMessage, AttachmentRepositoryImpl. introduced Mime4jAttachment.
added javadoc link to RFC 2231
fixed broken unittest
added TODO mark
fixed AttachmentRepository#readAttachmentPayload to read attachments with filenames encoded in character sets other than ascii and RFC 2231
introduced constant "application/json; charset=UTF-8"
removed System.out.println
Merge pull request #10 from ksokol/fix/attachment-umlaut

Fix/attachment umlaut