Adding plugin to import from Jekyll/Nikola sites

Hi there!

I've created a plugin to import Jekyll/Octopress sites to Nikola.

I've taken almost all code from my previous project https://github.com/magmax/jekyll2nikola
```py
from nikola.plugin_categories import Command, PageCompiler

```

```py

    def _serialize(self, in_metadata, doc, is_html):
        metadata = {'category': ''}
        metadata.update(PageCompiler.default_metadata)
        metadata.update(metadata)
        header = utils.write_metadata(metadata)

        pattern = '<!--\n{0}\n-->\n\n{1}' if is_html else '{0}\n\n{1}'
        return pattern.format(header, doc)
```

This:

 * renames `is_markdown` to `is_html` (both `.html` and `.md` posts use this mechanism which is actually HTML comments)
 * works more like `create_post` in the compilers:
   * uses default metadata from the page compiler class (guaranteed to be 100% correct); also adding `category` which will be added to the default set in getnikola/nikola#1533)
   * uses the standard `nikola.utils.write_metadata` function (which is different from what `import_wordpress` uses)
   * puts two newlines between meta and doc — we use this to remove the comment from the resultant HTML file
  * uses new-style string formatting, the de-facto standard in Nikola