Add 'avg' aggregate function

The 'AVG' aggregate function is specified in the SQL standard and
supported by MySQL, PostgreSQL, and SQLite. Include tests, too.
Document the shiny new 'avg' aggregate function
add pluck to builder interface

enables grabbing a single element from each row in results,
returning a promise which yields an array of those values.
Merge pull request #163 from benesch/feature-aggregate-avg

Add 'avg' aggregate function
Merge pull request #171 from tkellen/pluck

add pluck to builder interface
minor docs updates