Extend ordering with null sorting

[jwoertink]

fixes #153

This adds the ability to order with NULLS FIRST or NULLS LAST by adding an argument to the asc_order and desc_order methods.

Note this is a breaking change since the query builder now takes an OrderBy instead of some symbols. Though, the Query object will still take symbols since this is the one really exposed to users

# Optional null sorting
UserQuery.new.name.asc_order(:nulls_first)

# This still works
UserQuery.new.order_by(:name, :asc)

# This is gone
QueryBuilder.new(table: :users).order_by(:name, :asc)

[paulcsmith]

We should support sideways ordering in a future PR

[paulcsmith]

Love it!