Add solution and attachment data models #52
Bulk upload feature to populate textbooks #53

See Textbook#create action. Takes a zip file with directory structure
that corresponds to unit, chapter, section structure of questions.
parses solution HTML files and extracts attached images to upload.
resulting solution is displayed through the textbook browser. Remaining
issues: some images uploaded using Paperclip are empty files, even
though it seems to get extracted from the Zip file as a correctly formed
file.