Batch updates

[groue]

This pull request brings batch updates to the query interface:

// UPDATE player SET score = 0
try Player.updateAll(db, Column("score") <- 0)

// UPDATE player SET score = score + 10 WHERE team = 'red'
try Player
    .filter(Column("team") == "red")
    .updateAll(db, Column("score") += 10)

This change is directly inspired from stephencelis/SQLite.swift. Thank you!