Reorganizing tests.
Revamping encoding handling to normalize encodings to UTF-8 for storage; back into original encoding during rehydration.
Set email payload's charset after all other headers.
Cleaning up tests; moving test e-mail messages; fixing bugs relating to message encoding rehydration.

--HG--
rename : django_mailbox/tests/generic_message.eml => django_mailbox/tests/messages/generic_message.eml
rename : django_mailbox/tests/message_with_attachment.eml => django_mailbox/tests/messages/message_with_attachment.eml
rename : django_mailbox/tests/message_with_defective_attachment_association.eml => django_mailbox/tests/messages/message_with_defective_attachment_association.eml
rename : django_mailbox/tests/message_with_defective_attachment_association_result.eml => django_mailbox/tests/messages/message_with_defective_attachment_association_result.eml
rename : django_mailbox/tests/message_with_image_jpg_mimetype.eml => django_mailbox/tests/messages/message_with_image_jpg_mimetype.eml
rename : django_mailbox/tests/message_with_long_text_lines.eml => django_mailbox/tests/messages/message_with_long_text_lines.eml
rename : django_mailbox/tests/message_with_many_multiparts.eml => django_mailbox/tests/messages/message_with_many_multiparts.eml
rename : django_mailbox/tests/message_with_many_multiparts_stripped_html.eml => django_mailbox/tests/messages/message_with_many_multiparts_stripped_html.eml
rename : django_mailbox/tests/message_with_utf8_char.eml => django_mailbox/tests/messages/message_with_utf8_char.eml
rename : django_mailbox/tests/multipart_text.eml => django_mailbox/tests/messages/multipart_text.eml
Adding tests and minor code changes to ensure that we properly handle the distinction between bytes and text when dehydrating/rehydrating messages.