Add Dateish plugin

This plugin allows arbitrary metadata fields to be automatically
converted to datetime objects, just like the 'date' field.
Update Readme.md
Fix sitemap with disabled pages (fix #206)

Recommendation in pelican < 3.4 was to disable pages using
a False boolean (http://docs.getpelican.com/en/3.3.0/settings.html#basic-settings)
It's now recommended to use '' and the sitemap plugins tries to
join paths using those variables, which won't work with a bool.
This checks that we have a truthy value before joining.
Merge pull request #290 from Keats/master

Fix sitemap with disabled pages (fix #206)
Merge pull request #288 from wilsonfreitas/master

Improve rmd_reader plugin README
add footer_insert
update README
Handle subdirectories in thumbnailer.
Change MathJax URL from deprecated Rackspace CDN

Use new cdn.mathjax.org URL

More information available here:
http://www.mathjax.org/changes-to-the-mathjax-cdn/
Merge pull request #292 from isms/liquid-notebook-language

Change MathJax URL from deprecated Rackspace CDN
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.