Add size and PDF restrictions to downloads
reduce chunk size when downloading, raise exceptions on problems and close connection when done
Merge pull request #106 from CottageLabs/feature/add_size_and_pdf_restrictions

Add size and PDF restrictions to downloads
add plugin error class to record problems that plugins experience, and insert raises in doi and pmid detectors
only mine a page if it returns an OK status
Merge pull request #107 from CottageLabs/issue102

Issue102