remove ENCODING variable and hard code everything to utf-8

Appendum to #131. Assume that everything -- input and output files -- is
utf-8 encoded. Variable encoding is too error prone. Just use utf-8 and
everything is okay.
remove METASTYLE configuration parameter, related to #127

As proposed [1]

> * `METASTYLE` is currently the way to switch between (YAML front
> matter) and (reST, Markdown, pandoc). Actually, all file formats are
> very different and easy to distuingish. Therefore, I would like to
> remove the `METASTYLE` variable.

[1] http://librelist.com/browser//acrylamid/2013/2/20/review-remove-a-few-configuration-variables/
remove unused imports
don't hide traceback when excepting
set metadata from path, closes #132