Make FallbackMimePart interface match with MimePart

To achieve that common functionality has been extracted to
 RichPartMixin that is mixed to both classes.
Merge pull request #49 from mailgun/maxim/develop

Make FallbackMimePart interface match with MimePart
Make FallbackHeaders interface match with MimeHeaders
Merge pull request #51 from mailgun/maxim/develop

Make FallbackHeaders interface match with MimeHeaders
Ignore false Content-Type headers

It is ignored in mesage/part bodies as well as in multipart
preambles and epilogues.
Fix a typo in the RichPartMixin name
Merge pull request #50 from mailgun/maxim/tokenizer

Ignore false Content-Type headers in multipart epilogues
Merge branch 'master' into stable

Conflicts:
	flanker/mime/message/fallback/part.py
	flanker/mime/message/part.py
	tests/mime/message/fallback/fallback_test.py