Docs: Fix example in styleguide

Now the imports example conforms with the guidelines present a few lines above.
Output source when a unit is fuzzy

Fixes bug 3145
Don't use python-Levenshtein 0.11.0

A fix is here https://github.com/ztane/python-Levenshtein/pull/1

0.11.0 add Python 3 support, so not something we really need right now.
So until the fix lands, and since it is so close to a final release, let
us be conservative and make sure we're only using the working version.
Docs: Fix minor nit on styleguide example
Lets rather use our requirements files

Prevents situation where we have to fix things in multiple place i.e.
the python-Levenshtein breakage.
Docs: Small fixes in releasing notes
Refactor: Use a BasePlaceable class

This way it is possible to check if a random object is a placeable instance.
Docs: Change word in releasing docs
python-Levenshtein>=0.11.1

This fixes the issue we worked around in
88e6cf1df064578c951c6ee8baec98e52f388884 a new release solves the problem.
Improve handling of escapes in wrapping

In some cases we could still split an escape which gettext won't do. An
escaped newline should keep with text on its left.
Travis: Install the development requirements

Previously used requirements was not providing pytest nor Sphinx.
Cleanups
Cleanups
Minor change in Travis config
Docs: Toolkit born circa 2002
Release: retrieve the releasers sourceforge username
Improve po2tmx for cPO and fPO

Slight changes to code and tests to ensure that the extraction works for other
PO parsers.
PEP8: fix some tab whitespace errors
Extend NewlinePlaceable to support more cases
Exclude pulse audio when building XPI