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/oeb/transforms/metadata.py

@@ -1,9 +1,9 @@
+import mimetypes
 import os
 import re
 
 from ebook_converter.ebooks.oeb import base
 from ebook_converter.utils.date import isoformat, now
-from ebook_converter import guess_type
 
 
 def meta_info_to_oeb_metadata(mi, m, log, override_input_metadata=False):
@@ -156,9 +156,8 @@ class MergeMetadata(object):
         new_cover_item = None
         if cdata:
             id, href = self.oeb.manifest.generate('cover', 'cover.'+ext)
-            new_cover_item = self.oeb.manifest.add(id, href,
-                                                   guess_type('cover.'+ext)[0],
-                                                   data=cdata)
+            new_cover_item = self.oeb.manifest.add(
+                id, href, mimetypes.guess_type('cover.'+ext)[0], data=cdata)
             self.oeb.guide.add('cover', 'Cover', href)
         if do_remove_old_cover:
             self.remove_old_cover(item, new_cover_item.href)