Cleaned up test_sitemap_creation.php and fixed an off-by-one error.

While creating dummy posts, one post would be created with the day in the future. I subtracted one to ensure this does not occur.
Cleaned up a couple convulated ways of grabbing the last element of an array and replaced them with a call to end().
Added a means for counting the number of indexed URLs per sitemap and as a total.

In response to #50. Added an 'msm_sitemap_indexed_url_count' option that tracks the number of indexed URLs in every sitemap. When a sitemap is updated, the number of URLs it contains is updated. This statistic is printed out on the Create Sitemaps page of the plugin.
Removed an infinite loop in the find_next_day_to_process() that caused sitemap generation to always continue on the same day and other cleaning.

Cleaned up another strange method of grabbing the last element from an array.

Fixed an issue in MSM Sitemap where false would be inserted at the start of the msm_sitemap_indexed_url_count option.
Added UT coverage for sitemap statistics and fixed the existing UT.

Added a base class MSM_SiteMap_Test that implements some functionality like creating dummy posts and faking cron that all test cases require. The test classes could not inherit this class as it caused strange behaviour with WordPress Tests, so test classes just create a private instance of the class.

Added the test_sitemap_stats.php file to test statistics for post creation & deletion.

Fixed the test_sitemap_creation.php tests. Previously they could potentially provide a badly formatted date string for the post_date of a new post (eg '2012-01-8' -- note the lack of trailing zeros). To fix this I used the DateTime() class to format the date inside MSM_SiteMap_Test.
Sitemap stats now properly deleted when a sitemap is deleted.

Previously sitemaps stats did not get deleted when a sitemap got deleted, skewing stats.