Implement failure logging

[JimHokanson]

On failing to resolve references of a paper, or any other task, log the failure to disk to guide future enhancement work (i.e. supporting a particular publisher).

[ksarmentrout]

Logging is now implemented in pypub, within the GUI functions. CSV formatted lines are written to logfile.txt that include a timestamp, the method that threw the error, the error, the DOI of the paper being retrieved, and the DOI of the paper that references it (if relevant). Just pushed it up.

[JimHokanson]

Great! I'm hoping to get a chance to check it out soon! Once I do I'll close the issue

[ksarmentrout]

Right now I have it set up such that any error gets appended to the end of that one logfile.txt. I might want to change it to instead distribute into several files so the original logfile doesn't get too bloated and hard to look through. Not sure.