Add size and PDF restrictions to downloads

Just let me know if code looks reasonable. Feel free to pull down and run openarticlegauge/tests/run_content_plugin_tests.sh - they do pass for me. But the build as a whole was broken (there are some other failing tests) before these changes were introduced - there are simply some outdated tests in the suite.

Feel free to ignore the test_bmc.py file, I had to reformat it due to reported mix of tabs and spaces (?!). The mock_get function is copied in each test, and I needed to change it, that's why so many tests are touched. Not that easy to move it away either, since it's couple with some kind of CURRENT_REQUEST constant - so I just replaced the code in each test by hand *gasp*.