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.
This is progressing refactor of the calibre code to make it more
readable, and transform it to something more coherent.
In this patch, there are changes regarding imports for some modules,
instead of polluting namespace of each module with some other modules
symbols, which often were imported from other modules. Yuck.
