Sorted out mime initialization.

Every mime related function in main __init__.py has a flag check for the
check if initialization has already done. This is nonsense, since it
should be done implicitly early on the converter is starting.

This commit straight the things out, and initialization is done in cli
module.

Also, function guess_type was removed, since it's just a proxy for
mimetypes.guess_type function.

ebook_converter/ebooks/metadata/fb2.py

@@ -1,6 +1,7 @@
 """
 Read meta information from fb2 files
 """
+import mimetypes
 import functools
 import os
 import random
@@ -225,7 +226,7 @@ def _parse_cover_data(root, imgid, mi, ctx):
         mime_extensions = guess_all_extensions(mimetype)
 
         if not mime_extensions and mimetype.startswith('image/'):
-            mimetype_fromid = guess_type(imgid)[0]
+            mimetype_fromid = mimetypes.guess_type(imgid)[0]
             if mimetype_fromid and mimetype_fromid.startswith('image/'):
                 mime_extensions = guess_all_extensions(mimetype_fromid)