Added an enum for submission types and fixed the update URL.

Also changed assignment API to return needs_grading_count_by_section
Merge pull request #162 from bpluim/submission_fix

Added an enum for submission types and fixed the update URL.
updated podspec for new version
Adding previewURL to CKIFile
Merge pull request #163 from bpluim/previewURL

Adding previewURL to CKIFile
updated podspec for new version
Merge remote-tracking branch 'upstream/master'