Add Dateish plugin

This plugin allows arbitrary metadata fields to be automatically
converted to datetime objects, just like the 'date' field.
add footer_insert
update README
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.
Add GZIP_CACHE_OVERWRITE option to overwrite original files

This option is useful when hosting static content on a service such as
Amazon S3. Setting to True will overwrite the original file with
the gzip-compressed file. In order to correctly serve the file, the
Content Encoding must be set to gzip by the hosting service.
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
Correct list and code samples formatting
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.
Fixed #300. Sitemap plug-in: '%s' should be '%z' for printing out timezone.