Additional statistics for the Admin page, bug fixes, UI enhancements, fixing UTs

In response to #50, #53.

Stats: 
- Number of sitemaps
- Total number of indexed urls
- Number of indexed urls per sitemap.

Admin Page: 
- Removed Sitemaps CPT page as it was pretty useless in its current incarnation. See #53
- Renamed "Create Sitemaps" admin page to "Sitemap"
- Moved the code to output the admin page to msm-sitemap.php
- Halt Sitemap Generation button is only enabled when sitemap generation is running
- Added a status for when sitemap generation is being halted

Fixes: 
- Fixed a number of translation issues
- Sitemap generation always repeats for the current day (infinite loop bug)
- Added output escaping to the admin page

Unit Tests:
- Fixed existing UTs
- Added UT coverage for site stats