Grad handbook changes after AY18-19

[bwoshea]

Here are a bunch of changes to the grad handbook following the 2018-2019 academic year. The primary changes are:

1. Updated Master's degree section to clarify requirements regarding credits, GPA, 400-level course requirements, and minimum acceptable grades.
2. Updated PhD degree section to clarify requirements regarding credits, minimum acceptable grades, and 400-level course requirements.
3. Added text to Appendix A (Policy regarding prior graduate coursework) to explain how transferring graduate credits works and how credits for the Master's degree works.
4. A variety of typo fixes.

For changes 1-3, these changes are all either clarifications based on student questions or requirements coming from Graduate School policies. #4 primarily comes from things I noticed while working with individual graduate students.