Add Submission creation sketch to API spec.
Add list of concerns regarding Submissions
Remove bit about version including timestamp information.

Cale pointed out that if we want to do things by submission time,
we can always grab that information with submitted_at.
Remove is_active flag from submission posting.

We'll make a separate table that will track both the highest and
most recent grade for a given (course, user, problem, part) with
back pointers to submissions. So keeping an explicit is_active
flag is unnecessary.
Hints removed, question added to expand on some of the trickiness involved.
Merge pull request #1 from ormsbee/spec_submissions

Add Submission creation sketch to API spec - Haven't heard from anyone in a while, so merging. Please holler if you have concerns that were not addressed.