Added preproc script, and changes for multiple answer support, raw-scan ...

...PDF feedback, .signature in feedback email, and modified documentation accordingly

1. preproc.py script takes a tex file, and expands out all of the \input; this is useful for writing an exam.tex file that uses \input to import questions from a question bank, and produces a library file that can be used for randexam.

2. Changed randexam to support multiple answers per question from students, and partial credit. Introduces a "bubble" array that encodes all of the answers that a student has selected, and then grade / statistic / feedback accordingly.

3. Added support for a rawscan directory that includes PDFs of individual exams scanned in, that will be sent as an additional attachment to students in the feedback processing.

4. Added support to clarify the msg_txt for the email, and add in the ~/.signature file to the message, if present.

5. Edited documentation to include information about the multiple answer features and formatting of randexam.