Convert PDF files to PNG

When PDF files are uploaded, run pdftoppm to convert each page to a PNG
file. Delete PDF file afterwards.

Possible security implications when putting the command together. Also, files
are written directly to the Rails folder.
Get the list of converted PNG files and create individual scribing
questions for them.

"Empty" question is left from original PDF file. Questions are created
for each PNG file, but right now there is no way to see them.

PNG files are handled with the same mechanism (Paperclip gem, through
FileUpload model)

References Coursemology/coursemology.org#277
Change double quotes around string to single quotes as there isn't a
need to print a variable's value in the string

References Coursemology/coursemology.org#277
Merge pull request #3 from fonglh/scribing-qns