Bug #911 reimport detection for the `importadded` plugin

Reimports are now detected by inspecting the fields for reimported items
and replaced albums in ImportTask.

Reimported albums and items are skipped.
Comply with PEP8.
Add quoting and use displayable paths in log messages
Merge pull request #925 from silb/bug-911-importadded-reimport

Fixes bug #911
fix artresizer resize() method
Simplify Results.__iter__

Deduplicate some copypasta from the sorting feature.
dbcore.Results: Avoid duplicate construction

Iterating over a results set multiple times should not take the same amount
each time. We now keep around the materialized objects and re-use them with
iteration. This solves a performance problem in the `play` plugin, which uses
len(results) multiple times and was therefore taking an unnecessary
performance hit when the query was slow.
Second-order optimizations to Results getters

Fix #997. This provides shortcuts for len() and subscripting in certain
situations. There are of course more shortcuts that are possible, but this
will do for now!
duplicates: Fix #999 (check "checksum" type)
Fix #1006: errant info-level log

Introduced in e99df7bc65dc69d92dafb2df390812d0e2beeb3f.
Added handler for requests.exceptions.ConnectionError to discogs plugin.
Fix concurrent iterators in incremental results

Some weird behavior was possible when having two iterators on the same Results
object. May seem far-fetched, but it is possible. As an added bonus, this
saves a little memory by disposing of rows when they have been materialized
into model objects.
A bunch more tests for Results
Fiddling with Travis build

- verbose nose mode (easier to tell what's erroneously being skipped, fore
  example)
- less redundant installs
- travis_retry coveralls install
- separate py27cov tox environment; default tox runs will not use coverage,
  which can take a long time; use tox -e py27cov to get coverage numbers
Travis: Require sphinx

Oops. Looks like we need sphinx to run setup.py sdist for tox builds.
Travis: re-enable coverage

Use py27cov instead of py27.
A couple of coverage tweaks
Remove configuration migration code

Finally.
Merge pull request #1008 from otakucode/master

Added handler for requests.exceptions.ConnectionError to discogs plugin.
Changelog/thanks (fix #1004, #1008)