Fix handling of old and new-style MNRAS PDFs

This commit fixes Issue #29 where new (2013) MNRAS are showing
`UnicodeDecodeError` in `MNRAParser`. Overall, handling of both
new (Oxford hosted) and old (ADS hosted) MNRAS PDFs should be
fixed and improved.

Improvements for new MNRAS articles:

- `MNRASParser`: Detect and decode page's charset (UTF-8)
- Find `citation_pdf_url` in Oxford MNRAS HTML header. This is a
  different format from the Wiley HTML.

Older MNRAS articles (possibly the public domain ones?) have PDFs
hosted by ADS. By resolving the URL of the ADS article link we
can determine if the PDF is hosted by ADS, e.g.,

    if "filetype=.pdf" in resolved_url

or if we need to use `MNRASParser` to extract the url of a PDF
hosted by MNRAS itself.

The previous way of handling this could actually cause MNRAS entire
PDFs to be downloaded twice.
Merge pull request #28 from RuiPereira/work

Compare abstract before replacing publication.
bugfix: arXiv postings don't necessarily have comments
Merge pull request #30 from keflavich/no_comment_fix

bugfix: arXiv postings don't necessarily have comments