Adding filterable namespaces.

Added an `msm_sitemap_ns` filter during sitemap construction to allow
customization of namespaces similar to WP.com.

See #4
Closes #35
Added sitemap entry filter.

Added the filter `msm_sitemap_entry` which runs for each entry during
sitemap construction. The filter is passed the SimpleXMLElement for the
entry, however its return value is not used so this filter can only be
used to modify an entry, and not completely change or remove it.

See #4
Added a filter to skip a post during generation.

Added the `msm_sitemap_skip_post` filter that allows individual posts to
be skipped during generation. If a filter returns false the post will be
skipped.

This filter is called in the loop, so post data is not passed through the
filter.

See #4
Added a query args filter.

Added the `msm_sitemap_query_args` filter which allows for filtering of
the query args (an array) that are used to determine the posts retrieved
to be placed in a sitemap.

For information on the available query args see:
http://codex.wordpress.org/Class_Reference/WP_Query

See #4
Renaming the msm_sitemap_ns filter.

Renaming the `msm_sitemap_ns` filter to `msm_sitemap_namespace` for clarity in response to comments received on 1f66b98303.
Merge pull request #56 from Mobius5150/moar_filters

Adding Moar filters