Basic import from Blogger export

It's possible to export an entire Blogger blog into a single Atom XML. 

The 'feed' importer does a pretty good job at this, but it gets a few things wrong:

* comments & settings show up as posts
* slug doesn't match the blogger slug
* it encodes the 'kind' as just another tag
* it doesn't identify pages

This importer addresses those issues.

It doesn't try to clean up Blogger's (usually awful) HTML. 

I had the best results running it with `--wp-custpost` (to separate out settings, templates, and comments) and `--dir-page`, like this:

    pelican-import --blogger -m markdown -o $OUTPUT_DIR --wp-custpost --dir-page blog-08-29-2013.xml

At least on my blog, the generated rST is useless.