Main improvements (see also merge from #20)

• Check for server error status codes when validating on dx.doi.org as 504 errors can occur
• When performing google searches, it looks for DOIs also in the URLs.
  • Support any URL with a matching DOI and the doi keyword in the URL.
• Attempt to strip extensions from filenames doi10.111/1111.pdf will fail to locate the doi as 10.111/1111.pdf is a valid, if uncommon DOI.
• "Standardise" DOIs to handle loose matches e.g. case variations, or trailing punctuation.

Minor code changes (see also merge from #20)

• Moved regex patterns to patterns.py + add pytest tests for common DOI patterns
• Update to use logger.exception which provides tracebacks on errors.
• Moved code to add the '/identifier' tag to a general function add_metadata() in finders.py