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
Support include regex in thumbnailer
Gallery: sort image files by name
Merge branches 'include-regex-for-thumbnailer' and 'sort-by-name-in-gallery' into blog
Merge pull request #1 from ntj/blog

Blog