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/jacket.py

@@ -1,3 +1,4 @@
+import mimetypes
 import sys, os, re
 from xml.sax.saxutils import escape
 from string import Formatter
@@ -5,7 +6,7 @@ import pkg_resources
 import urllib.parse
 
 from ebook_converter import constants as const
-from ebook_converter import guess_type, strftime
+from ebook_converter import strftime
 from ebook_converter.constants_old import iswindows
 from ebook_converter.ebooks.oeb import base
 from ebook_converter.ebooks.oeb.base import XPath, xml2text, urlnormalize
@@ -124,7 +125,8 @@ class Jacket(Base):
                 alt_comments=comments, rescale_fonts=True)
         id, href = self.oeb.manifest.generate('calibre_jacket', 'jacket.xhtml')
 
-        jacket = self.oeb.manifest.add(id, href, guess_type(href)[0], data=root)
+        jacket = self.oeb.manifest.add(id, href, mimetypes.guess_type(href)[0],
+                                       data=root)
         self.oeb.spine.insert(0, jacket, True)
         self.oeb.inserted_metadata_jacket = jacket
         for img, path in referenced_images(root):
@@ -132,7 +134,9 @@ class Jacket(Base):
             ext = path.rpartition('.')[-1].lower()
             item_id, href = self.oeb.manifest.generate('jacket_image', 'jacket_img.'+ext)
             with open(path, 'rb') as f:
-                item = self.oeb.manifest.add(item_id, href, guess_type(href)[0], data=f.read())
+                item = self.oeb.manifest.add(
+                    item_id, href, mimetypes.guess_type(href)[0],
+                    data=f.read())
             item.unload_data_from_memory()
             img.set('src', jacket.relhref(item.href))