update readme with requirements

better_figures_and_images doesn't make it clear in the readme what
requirements are required.
Merge pull request #99 from fly/better_requirements

update better_figures_and_images readme with requirements
added the ability to override\force related_posts from the post's meta data
Merge pull request #102 from Shaked/master

Override related_posts from post metadata
Update sitemap plugin to place nicely with modified metadata

modified is a string in pelican <= 3.3 and datetime > 3.3

Sitemap used to treat it as string.
Merge pull request #103 from talha131/fix-sitemap

Update sitemap plugin to play nicely with modified metadata
Refactor and fix minor typo in sitemap code
Merge pull request #104 from talha131/cleanup-sitemap

Refactor and fix minor typo in sitemap code
Minify JSON to improve search speed
Merge pull request #105 from talha131/minify-json

Minify JSON to improve search speed