Handle subdirectories in thumbnailer.
Update bootstrap_rst_directives.py

changed alert-default to alert-warning in docs, there is no 'alert-default' in bootstrap.
ADD: sort_tags plugin

This plugin adds `tags_sorted_article_length` to the context,
which is a list of tupels (Tag, [Articles]) that is sorted 
by number of Articles first and Tag second.
Update Readme.md
Fix gallery plugin crash. Use correct loop variable name.
Prevent compression of webm video files.
Merge pull request #293 from trvrm/patch-1

Correction for bootstrap_rst_directives.py docs
Merge pull request #297 from adworacz/fixGzipWebm

Prevent compression of webm video files in gzip_cache.
Merge pull request #295 from jmduke/patch-1

Minor fixes for Readme.md
Merge pull request #296 from bjornhol/master

Fix gallery plugin crash. Use correct loop variable name.
Merge pull request #294 from ingwinlu/patch-1

Add sort_tags plugin as submodule
generating the temp_article now uses empty string rather than None
Merge pull request #301 from alistairmagee/bugfix

Compatibility fix for Clean Summary and temp_article plugins
Merge pull request #305 from clokep/thumbnailer-subdirs

thumbnailer plugin: Handle subdirectories appropriately.