Fix --output-file option

When opening the file for binary writing, the file mode was incorrectly
specified.

Additionally, the command line options have been updated to be more
intuitive:
- long: `--report-file`
- short: `-r`

The file can now be successfully written with:

python create_pdfs.py \
    --report-file report.csv \
    --course-id 'edX/DemoX/Demo_Course' \
    --no-upload \
    --name 'Guido'
Merge pull request #17 from stvstnfrd/output-file/mode

Fix --output-file option