Add support for Flat XML OpenDocumentFormat files

[jonadem]

When I try to open a .odt file, I get:

>>> textproc = ezodf.opendoc("foo.fodt")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/ezodf/document.py", line 54, in opendoc
    mime_type = __detect_mime_type(fm)
  File "/usr/local/lib/python2.7/dist-packages/ezodf/document.py", line 69, in __detect_mime_type
    mime_type = MIMETYPES[ext]
KeyError: '.fodt'

Same for .ods files

[jonadem]

I think commit c96fe52 introduced the issue.
Before it checks first if provided file is a zip; after it checks first if filename is not None (and don't reach the flat xml test).

[jonadem]

PR #23 solve the issue.