feat(limit_by): Add support for LIMIT BY clause

[jdenquin]

• LIMIT 1 BY field clause is a very performant alternative for de-duplication. It's better than a FINAL or GROUP BY on large sets.
• Add the support of this clause for the gem

Usage :

# limit_by(<Integer>, <Symbol> or <String>)
events = Event.limit_by(1, :event_key)

• NOTE: you can't use aggregate functions with a LIMIT BY without grouping, if you want to do so, you should use CTE, eg:

events = Events.limit_by(1, :event_key)
sql = <<-SQL
  with events as (#{events.to_sql})
  SELECT count()
  FROM events
SQL
Events.connection.select_value(sql)

• TODO: Add support for a list of fields limit_by(<Integer>, <[Symbol]> or <[String]>

[PNixx]

@jdenquin please, fix conflicts

[jdenquin]

@jdenquin please, fix conflicts

done!