Add blogname to WXR export

[RyanMurphy]

From the Ideas section: ​http://wordpress.org/extend/ideas/topic/append-exported-wordpress-xml-file-with-site-name

I have few website's I administer and I download the export file, just in case, once in a while, (I do do a full db backup everyday as well.) and find it annoying that the default file name for the .xml is wordpress.todays-date.xml. How about making it my-site-wordpress.todays-date.xml?

[nacin]

(In [15961]) Importer and exporter overhaul, mega props duck.

Exporter overhaul:

• Add author information to export
• Greater usage of slug identifiers
• Don't export auto-drafts, spam comments, or edit lock/last meta keys
• Inline documentation improvements
• Remove filtering for now (@todo)
• Bump WXR version to 1.1, but remain back compat in the importer

Importer overhaul (​http://plugins.trac.wordpress.org/changeset/304249):

• Use an XML parser where available (SimpleXML, XML Parser)
• Proper import support for navigation menus
• Many bug fixes, specifically improvements to category and custom taxonomy handling
• Better author/user mapping

Fixes #5447 #5460 #7400 #7973 #8471 #9237 #10319 #11118 #11144 #11354 #11574 #12685 #13364 #13394 #13453 #13454 #13627 #14306 #14442 #14524 #14750 #15055 #15091 #15108.

See #15197.