Use new export API

The current code that implements `wp export` is a mess, because the WordPress export code is a mess.

@nb proposed a proper export API (he says it's already being used on wordpress.com).

Ticket: http://core.trac.wordpress.org/ticket/22435
Diff: https://github.com/nb/WordPress/compare/master...export-api

If we namespace it properly, we can start using it before it lands in Core.

Tasks:

* [x] do something with the `--verbose` flag
* [x] replace `VerboseExportWriter` class with action inside `WP_Export_Split_Files_Writer`
* [ ] add parameter for specifying filename format [#issuecomment-20013095](#issuecomment-20013095)
* [ ] call `export_wp` action again?
* [ ] manually test with large export