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/reader.py

@@ -4,6 +4,7 @@ Container-/OPF-based input OEBBook reader.
 import collections
 import copy
 import io
+import mimetypes
 import os
 import re
 import sys
@@ -22,7 +23,7 @@ from ebook_converter.utils.cleantext import clean_xml_chars
 from ebook_converter.utils.localization import get_lang
 from ebook_converter.ptempfile import TemporaryDirectory
 from ebook_converter.constants_old import __appname__, __version__
-from ebook_converter import guess_type, xml_replace_entities
+from ebook_converter import xml_replace_entities
 from ebook_converter.polyglot.urllib import unquote
 
 
@@ -130,7 +131,6 @@ class OEBReader(object):
             meta_info_to_oeb_metadata
         stream = io.BytesIO(etree.tostring(opf, xml_declaration=True,
                                            encoding='utf-8'))
-        # o = opf_meta.OPF(stream)
         o = OPF(stream)
         pwm = o.primary_writing_mode
         if pwm:
@@ -251,7 +251,7 @@ class OEBReader(object):
                                      href)
                     warned.add(href)
                 id, _ = manifest.generate(id='added')
-                guessed = guess_type(href)[0]
+                guessed = mimetypes.guess_type(href)[0]
                 media_type = guessed or base.BINARY_MIME
                 added = manifest.add(id, href, media_type)
                 unchecked.add(added)
@@ -268,7 +268,7 @@ class OEBReader(object):
             if media_type is None:
                 media_type = elem.get('mediatype', None)
             if not media_type or media_type == 'text/xml':
-                guessed = guess_type(href)[0]
+                guessed = mimetypes.guess_type(href)[0]
                 media_type = guessed or media_type or base.BINARY_MIME
             if hasattr(media_type, 'lower'):
                 media_type = media_type.lower()