syspath: correctly prefix Windows UNC paths

Identified while tackling #670, but this should actually solve some legitimate
problems with cataloging music on a network drive.
skip pyechonest tests when library not installed
more test isolation (#670)

Doing test-specific cleanup in tearDown before general sandbox deletion helps
avoiding contamination of global state between tests when cleanup fails.

Current Windows status:

    Ran 1106 tests in 72.373s
    FAILED (SKIP=10, errors=13, failures=15)

Closer!
flake8 cleaning beetsplug/*

Remaining warnings are related to visual indentation.
Merge branch 'KraYmer-flake8'
restore some comments from #684

I wanted to leave a few of these as comments -- they serve more as separators
than as internal documentation.
fix up some indentation warnings in plugins
flake8-cleanliness in missing

This is a little bit dumb, but one way to get the style checker to accept the
nice alignment in this plugin is to make it a dict.
`beet modify`: Treat arguments with : before = as queries, not modifications

This can be useful if the value being queried contains an equal sign.
flake8 cleanup in library and functemplate
avoid extra output in test mp3gain run
enable unused import checking

The "noqa" marker is for exceptions such as these.
`beet modify`: Add simple argument parsing tests.
fix #685: truncate files larger than 50MB for upload

The new behavior is as follows:

1. If the file size is greater than 50MB, and the 'truncate' config
option is 'no', skip the file.

2. If the file size is greater than 50MB, and the 'truncate' config
option is 'yes', reencode the file to ogg, and truncate to first 5
minutes.
call truncate, not convert, when truncating
fix #687: template indexes in logging statements
Merge pull request #686 from mook/master

`beet modify`: Treat arguments with : before = as queries, not modifications
changelog & style tweaks for #686
commands.py is flake8-clean
standardize placement of command setup

This groups together all of the optparse setup calls separately from the
functions. This comes at the expense of showing the command-line options above
the code that interprets them.

Making me wish for a more declarative CLI setup style...