Remove ease-of-use functions from the DB type.

Functions such as DB.Put(), DB.Get(), and DB.Delete() were originally
added to be easy to use, however, after implementing Bolt in multiple
projects I have found these ease-of-use functions useless. Nearly
every use case requires multiple calls in a single transaction.

Using the DB ease of use functions turned out to be an antipattern.
Merge pull request #68 from benbjohnson/remove-db-functions

Remove ease-of-use functions from the DB type.