Improvements to DBQuery help(), comment() and some internal refactoring

I opened PR#270 some time ago (probably during 2.2's QA). It includes a few small improvements to DBQuery's JS:

• DBQuery.comment() now returns the DBQuery object instead of chaining to this.next() (consistent with other methods, although maybe this is considered a BC break?)
• comment(), snapshot(), and readPref() were added to help output
• Use this._addSpecial() more to remove some code duplication

If necessary, I can break these apart into smaller commits and/or remove some changes.