mpdstats: update last_played

This fixes #529 and brings back the old behaviour of setting last_played
on every song change regardless of whether it is played to the end or skipped.
echonest: use only one plugin class (fix #536)
Fix test run with setuptools
restore echonest fetch command (fix #540)
lastgenre: tolerate `el.weight is None`
Merge pull request #535 from kljohann/last_playing

mpdstats: update last_played
changelog/thanks for #535
setup.py: remove namespace_packages declaration

Looks like the Setuptools people are moving to make this an error. I don't
think it was doing anything anyway.

https://bitbucket.org/pypa/setuptools/issue/12/update-or-remove-warning-when-namespace
Revert "Replace path separators from config"

This reverts commit c82b31e750995596e9c50173750b593402895a2c.

Conflicts:
	docs/changelog.rst
clean up --config handling

In reference to a363d9672f2be9de44cd1d3b3ae1fa1e69c1c5c9. This helps preserve
modularity/readability by avoiding contamination of the subcommand logic with
beets-specific logic.
fix testability of main entry point
changelog for config priorities (#513)
modify: remove special parsing logic

This could be nicer, but at least we now use standard type-based parsing.
Model._parse() class method
modify: use formatted values in _showdiff()

Perhaps we should use this for other commands also. One outstanding issue is
strange highlighting (e.g., "True -> False" helpfully shows that the "e" did
not change).
link to copyartifacts plugin
date queries: light style changes
add some failing tests for date queries
add TOCs to some documentation
fix numeric queries over empty string (fix #547)