Common terminology for review comments

@singingwolfboy @sarina @nedbat Please take a look at the suggested terminology and definition proposed in this PR.  I spoke to @singingwolfboy about the benefits of having a common vocabulary for this and he proposed creating this PR to discuss it.  I'd appreciate your feedback.  I find this will be useful within our T&L team and possibly for edX at large.
@cahrens FYI.
Considering the definition of nitpicking (http://www.oxforddictionaries.com/us/definition/american_english/nitpicking), I was not sure whether to actually include this category since it implies the comment is miniscule and unimportant in the grand scheme of things.  However, it seems reviewers still frequently use the word "nit" when they think their comment could be perceived as being picky, and could potentially be a religious debate (or not - if there are clear guidelines).

So how about:
```
A comment tagged with "Nitpick" indicates the reviewer has a petty criticism that *may* not be critical to address, but considers important to share in the given context.  Contributors should still seriously consider and weigh these nits and address them in the spirit of maintaining high quality code.
```

I am also open to removing the "Nit" label altogether if it simplifies things.  That is, if "nit" indicates the size of the change (tiny) and doesn't give input on the necessity of the change ("must" or "optional").