Moving per-sitemap url counts to post metas.

Previously the total number of indexed URLs for each sitemap was stored in
a single option `msm_sitemap_indexed_url_count`, which ran the risk of
becoming much to large on sites with many sitemaps.

This change moves the count of urls in each individual sitemap to a
`msm_indexed_url_count` post meta for that sitemap.
`msm_sitemap_indexed_url_count` now hold the total number of indexed URLs.

Updated msm-sitemap.php and wp-cli.php to reflect these changes.

One side effect of this is that it is no longer possible to retrieve the
count for every sitemap on the site using `get_indexed_url_count()` as
that would require a large number of queries. Instead, this function is
now passed the year, month and day to retrieve.

Closes #60.
Fixed incorrect use of get_post_meta().

`get_post_meta()` was used as if the third argument is the default value,
whereas it is actually whether to return a single (string) value.

This fixes a bug where the number of indexed urls gets incremented every
time generation runs.
Properly removing all site stats and generated XML when sitemaps reset.

If the sitemap stats stored in the post meta are not removed when sitemap
data is reset, then they will be forever buggered up even if a manual
generation and counting is done.
Outputting better message when sitemap stats reset.

Previously the message for when sitemap stats were reset instructed users
to manually remove remaining data, but did not say how. Updated the
message to include the CPT to remove as well as the wp-cli command to do
the removal.
Fixing incorrect post_meta name and use.
Updating UTs for the new sitemaps stats.

The method `get_indexed_url_count` changed in 8c1d7ab. Updating stats to
reflect this.

Added a helper function to `MSM_SiteMap_Test` that checks that the stats
are correct for all posts created using `create_dummy_posts()`. This
removes duplicate code for stat checking.

Note that the `test_site_stats_for_deleted_post()` test is currently
failing because sitemaps are not being removed when all posts from that
day are deleted. This test is failing correctly and the issue should be
fixed in another commit.
Refactored code from grabbing sitemap id into function.

Created a `get_sitemap_post_id` function that gets the WP post id for a
sitemap with a given date as this code was being used in several places.

From comments received on #64
Returning false instead of 0 in stats functions if value not found.
Updating references to get_sitemap_post_id() to check false instead of 0.
Merge pull request #64 from Mobius5150/master

Move per-sitemap url counts to post metas