#1530 - move scoring to the correct tables.

Removes appropriate:
ExamRecordScore
ExamRecordScoreField
ExamRecordScoreFieldChoice

Creates new:
ExamRecordScore
ExamRecordScoreField

Updates score in:
ExamRecord
ExamScore

Also, I replaced the colons in the export file names with dashes as
colons are not good in Windows file names.
Merge branch 'master' of http://github.com/Stanford-Online/class2go into
dca_new_working2

Conflicts:
	main/courses/exams/views.py
Grade export files contained a blank line at the end that was causing an
warning on import. Removed the blank line if file was going to be larger
than zero size. The blank line is still needed for otherwise zero length
files as S3 throws an error.
Merge pull request #1733 from Stanford-Online/dca_new_working2

Move CSV scoring to the correct tables #1530