update "custom" output for functional tests

LC_ALL=C pelican -o tests/output/custom/ -s samples/pelican.conf.py samples/content/
Merge branch 'pr/551' (closes #551)
Merge branch 'pr/555'

Conflicts:
	pelican/contents.py
update changelog: blank author are now allowed

(refs: #555)
docfix: fix default values for _URL and _SAVE_AS settings

(AUTHOR_URL, AUTHOR_SAVE_AS, CATEGORY_URL, CATEGORY_SAVE_AS)
add preread signal
Mention that FILES_TO_COPY is able to copy dirs.
What would be a commit without his "fix typo" brother, uh?
Merge pull request #539 from bbinet/recursive-diff

directory comparison should be recursive in functional tests
add docs for article_generate_preread signal
Move webassets tests in their own file.
Add a new signal `generator_init`, invoked at the end of `Generator.__init__`.
Use the new generator_init signal for the assets plugin.

This allows to have the assets_environment in all generators.
travis configuration for webassets tests: install webassets, cssmin and sass.
Change webassets configuration for relative urls.

Set the ASSET_URL to be relative to the 'theme/' url, which requires to use
{{ SITEURL }}/{{ ASSET_URL }} in the template and make it works with both
relative and absolute urls.
Kill SimpleHTTPServer also on OS X
delete the TODO file
Merge pull request #583 from egh/master

create a preread signal
remove old import statement
List of translations for simple and notmyidea pages