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.