Validate that score properties are numbers.

[garemoko]

It'd be nice if TinCanJS ensured that all the score properties were numbers, not strings.

See https://github.com/LearningLocker/learninglocker/issues/113

[brianjmiller]

This would be a fairly significant change, again because of the lack of "setter" model. To date we haven't validated types of any of the values being set, except for where they are versioned and then only through run time exceptions.

Though we can certainly leave it open as a to do, and 1 more nail in the need for proper accessors.