Queries cannot be modified once created

[shadowhand]

Consolidating discussion from #71, #66, #58, #31 to document the plan for implementing modifications to queries. This applies mainly to SELECT but also other queries.

WIP: This issue will be modified until a clear plan is made.

Currently some methods in Latitude are additive and some are setters. For instance, columns() always changes the column list and join() always adds a new join. A full list:

method name	add	set
columns()		x
addColumns()	x
from()		x
addFrom()	x
join()	x
where()		x
andWhere()	x
orWhere()	x
groupBy()	x
orderBy()	x
limit()		x
offset()		x
having()		x

Not all methods need to be additive and not all methods need to be setters. It should be possible to reset values that are additive.