youtube/vimeo/video tag improvements

- Allow more flexible video identifiers, current one "\w" blocks e.g.
youtube identifier "ul-FqOfX-t8". New one allows also attaching
positional arguments to video, e.g. "ul-FqOfX-t8#t=123"
- Surrounded each video element with a div having "videobox"-class for
styling purposes (e.g. center alignment)
- improved code formatting
import AsciiDocReader from core as a plugin
Added Image metadata attribute for article to specify a custom representative image.
Add unit test for custom image
Update readme to clarify behaviour of plugin and specifying a custom image
Merge pull request #218 from jkrshw/feature/custom-image

[representative_image] Add Image metadata attribute for article to specify a custom image
added check if PIL can open/process the current file and skip if not
reason: if there are any non-image-files (e.g.*.db) inside the image-folder site generation fails
Update gzip_cache to produce consistent .gz files.

The gzip protocol includes the timestamp in the file
by default. This causes the contents of the file to
be slightly different every time, causing problems
for some long-term caching strategies.

This patch removes the timestamp, allowing gzip to produce
the same file from the same source every time.
Merge pull request #226 from pmclanahan/deterministic-gzip

Update gzip_cache to produce consistent .gz files.
Merge pull request #220 from avaris/asciidoc_reader

Extract AsciiDocReader from core into a plugin
[pelican_comment_system] Fixes wrongly reinitialization, updates requirements list and feeds can now be disabled

Fix:
 - Fixed wrongly reinitialization in combination with the upcoming redesign of the i18n_subsite plugin
 - Added pelican 3.4 to the requirements list.

Feature:
 - Comment feeds can now be disabled
Merge pull request #223 from Scheirle/pcs_i18n_fix

[pelican_comment_system] Fixed wrongly reinitialization + updated requirements list.
Merge pull request #225 from znegva/thumbnailer-ignore-nonimages

improved thumbnailer plugin
Fixes #193 Adding TEMPLATE_PAGES to tipue_search plugin
Fix outdated link to Pelican docs
Merge pull request #194 from Futurile/template-pages-search

Add TEMPLATE_PAGES to tipue_search plugin. Fixes #193
[tipue_search] Restore support for Python 3
Merge pull request #230 from Schnouki/patch-1

[tipue_search] Restore support for Python 3
Merge pull request #207 from jojanaho/master

Liquid tags: youtube / vimeo / video improvements
PLUGIN_PATH setting has been replaced by PLUGIN_PATHS and it should be a list