Use VError && VError.info under the hood #76

The context option was implemented before verror's info option was available. Now that it's available upstream we should try and use that under the hood instead. Some implications in terms of how to access the context property and inspect it (verror uses a static method verror.info()). Should be able to continue to use the same serializer, however, since it already supports verror's info field.

Replaces #55