Pubmed-lookup

- Refactor tests
- Test are requiring biopython>=1.66 to run/pass
- Fix error message for Python (3.3+) version check
- Remove Python 3.3 and 3.4 from Travis CI config

- Accommodate publications with no authors
- Retain DOI URL when resolving throws URLError
- Catch connection reset errors when resolving DOI URLs
- Indicate compatibility with recent versions of Python 3 (through 3.7)
- Update README badges

- Change import of ``pubmed_lookup.command_line`` that was causing Travis CI to fail

- Create command-line tool (``pubmed-citation``) to quickly retrieve citations for PubMed IDs/URLS
- Create command-line tool (``pubmed-url``) to quickly retrieve article and DOI URLs for PubMed IDs/URLS
- Add option to save time by not resolving DOI URL (``Publication(pubmed_record, resolve_doi=False)``)
- Clean up documentation

- Add exception for publications that do not have month information (Thanks to Sasha Cuerda!)

- Resolve PEP8 errors
- Refactor code to be cleaner
- Configure Landscape and add code health badge
- Convert Github installation instructions to install from develop branch