Corpus & Article updates, pytest, allofplos.update

0.11.0
New Features
– Article class properties: Taxonomy, Creative Commons license, volume, issue, elocation, doi_link
– Many new Corpus class utilities with tutorial Jupyter Notebook, including len(Corpus()),for article in Corpus(),Corpus.random_article
– Deprecates allofplos.plos_corpus in favor of allofplos.update

Fixes
– Removes all reference to PLOS's internal URLs
– URLs now journal-specific; can construct each type of article link (to XML, to landing page, etc)
– Tests no longer require internet connection, but do require pytest
– Update readme and instructions for devs
– Reorganize corpus code
– Fixes Article class properties: abstract, figure-count, table-count, journal
– Fixes bugs in transformations & download functions