Importer generates too short header line.

Situation: pelican-import command converts wordpress xml to ReST text.

It seems that header line length depends on title character length, but pelican generates following warning in html.

```
System Message: WARNING/2 
(<path_to_content>/content.rst, line 2)
Title underline too short.
```

Because some languages (ex. Japanese) used wide character. 
```#``` line should  appropriately lengthened.
Instead of using len(), use unicodedata.east_asian_width to calculate correct title length.