Escape loc url (#103)
Prevents warnings/failures when a permalink for a post has an entity like `&`:
Warning: SimpleXMLElement::addChild(): unterminated entity reference utm_medium=xyz&utm_campaign=abc in msm-sitemap/msm-sitemap.php on line 446
Merge pull request #97 from mdbitz/master
Unit Test update to resolve run time and concerns with current logic
Use `c` as the date format for lastmod (#101)
The format we introduced in #94 was a bit broken.
props @nschoenholtz and @romancandlethoughts