fix missing 'kind' arg in importer

fixes #983
Remove old mercurial files
Deliberate overriding of an existing file to tests

Deliberate overriding via `save_as` metadata should be allowed, even after the
overwrite detection feature. This commit is to add tests for deliberate
overriding. As a result, the relevant tests *should fail* after this commit.

Added a page and an article, both to override a tag, with very old dates so
it limits the amount of diff in the generated pages.

Overriding feature introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72
Overwrite detection introduced by ff7410ce2ada85b486a67ae11874d60d135ff939
Delib. overriding and overwrite detect. Fixes #938

Make deliberate overriding (*) works with overwrites detection.

(*) first introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72

The following are decided to be deliberate override:
 - articles using the `save_as` metadata
 - pages using the `save_as` metadata
 - template pages (always)

Pelican now exits in the following 2 cases:
 - at least 2 not deliberate writes to the same file name (behaviour introduced
   by the overwrite detection feature ff7410ce2ada85b486a67ae11874d60d135ff939)
 - at least 2 deliberate writes to the same file name (new behaviour)

Also added info logging when deliberate overrides are performed.

Switched to StandardError instead of IOError, thanks to @ametaireau and
@russkel.
Use the `docutils` variable to check if docutils is installed, not `core`.

+ remove duplicate import
Replace get_populated_generator with setUpClass
More refactoring for test_generators

- list comprehensions for distill_articles/pages
- distill articles only once
- pep8
Refactor pelican_open using contextmanager
Merge pull request #943 from Rogdham/override_save_as_url

Deliberate overriding and overwrite detection. Fixes #938
Add warnings for img with empty alts. Fixes #647.

Unify Rst and Mkd: if alt is not specified at all, create an empty alt.
Warns the user in case of empty alts.
Move PDF generation to a plugin. Fixes #1009
Merge pull request #1003 from russkel/tumblrfix

fix missing 'kind' arg in tumblr importer
Merge pull request #1004 from russkel/hgrm

Remove old mercurial files
Merge pull request #1010 from kylewm/pdf-plugin

Move PDF generation to a plugin (Fixes #1009)
Merge pull request #895 from Rogdham/empty_alt

Add warnings for img with empty alts. Fixes #647.
Merge pull request #1006 from saimn/fix_docutils

Use the `docutils` variable to check if docutils is installed, not `core`
Remove the reference to the PDF generation in the docs. Fix #1013
Merge pull request #1007 from saimn/refactor-tests

Refactor test_generators and pelican_open
fix py3 support for sourcecode directive

added sourcecode to the functional test so it's tested *somewhere*.
fixes #963