Allow viewing on a local iPad during development.

ApplicationController#redirect_to_https was redirecting on any device
that tried to access the site in development (unless it was coming from
localhost or 127.0.0.1). This change allows remote devices to navigate
to the site using the development computer's IP address.
Fix the bug introduced in #258

The bug that was occuring here was a result of assignment_categories
with no assignments in them. In order to remedy this, we updated
GradeCalculator to specify a starting value for the map over each
category (re: `reduce(0, :+)`). This was the exact same bug that occurred
in #253 that was subsequently fixed in #256. (Note to selves, if using
reduce with an array that may be empty, __specify the starting value__).

As we were rounding grade all over the place, `Student#grade_in` now
rounds the grade to 1 decimal point automatically.

`students/courses/show` was using the legacy
`Student#calculate_percent`. Updated that.

To finish off the commit, we added a test that makes sure the
GradeCalculator can handle the error condition that existed before
(e.g. empty courses/assignment_categories). This involved adding an
identity to the Student factory.
Standardize grade rounding to a single decimal place

All over the app, grade rounding was being done to several different
units of precision. Per a discussion with @rwarbelow, this commit
unifies them all to one decimal place.
Merge pull request #260 from rwarbelow/fix-eric-and-tjs-mistake

Fix calculate_percent mistake