[Improvement] --pretend option for the convert plugin

Partially resolves #877 showing:
  - Directory creation
  - Copies
  - Deletes
  - Moves
  - Encodings

Information about tagging and plugins on _after_convert_ is not
currently shown. That requires changing the plugins to support the
pretend option, so a lot of work may be needed and it doesn't seem to be
helpful enough for me.
[PEP8] I didn't had a pep8 checker on vim :c
Merge branch 'master' of https://github.com/sampsyo/beets

Conflicts:
	beetsplug/convert.py

PS: I O U the logging changes, it's quite late :c
Docs tweaks
Merge pull request #891 from Dietr1ch/master

[Improvement] --pretend option for the convert plugin
Clean up `convert --pretend` (#891)

There were a number of problems with the changes to the util melange:
- It used print rather than logging, and its string formatting was probably
  not Unicode-ready.
- The shell-command-like print lines were not quite compatible, which makes
  their general usefulness questionable.
- Used an unsafe/leaky global variable for mkdirall.
- Used deprecated sets.Set.
Seemed better just to add this to the plugin where we need it so it's easier
to see where this goes.

It also seems unnecessary to me to print `mkdir -p` commands. They just
clutter up the output for me when I really just want to see the transcoding
commands.
Docs/changelog for #891