[Feature] Added new feature to allow union

Feature added to select query builder.

squel.select().field('field1').from('table1').union(squel.select().field('field1').from('table2'))

Updates:
> A new UnionBlock has been created and added to the end of the select query builder.  Must be at the end to ensure the union statement occurs at the end of the first query.
> toParam() for numbered parameters has been catered for by adding a new option that is passed to the sub-select query builder.  The nestedBuilder option is used internal to the builder to only update the query parameters with numbered parameters if it is the parent query builder.
> Test cases added to select.test.coffee and postgres.test.coffee for union and toParam calls.
[Feature] Union support fixes after rebase

Fixed a problem with the tests failing after an rebase to master.  For some reason the SELECT query builder had no knowledge of the union and union_all methods. Not entirely sure why I had to add the SELECT query builder to postgres.coffee?