Single quote alias names

Adds single quotes to SQL of the form `AS foo`, e.g. `@db[:foo].select{bar.as(baz)}` is now rendered as `SELECT bar AS 'baz' FROM foo`

Addresses https://github.com/colinmarc/sequel-impala-adapter/issues/5 because impala will tolerate column names that collide with functions as long as the column name is single-quoted.