support peer reviews & other utility changes

[x-j]

This is pull request #142

I was using the allofplos module in order to find peer-reviewed articles in the PLOS database. For this purpose I extended the functionalities of the Article class: added functions for obtaining sub-articles, keywords, a new constructor etc.

I also modified a few regexes, the plos_page_dict and some validating functions in order to include PLOS urls that host peer reviews.