Support subquery in FROM operator
Add UNION operator
Fix license line
Created SubQueryBuilder for defining subquery.

fix for review comments.
Simplified godoc for SubQueryBuilder.

to reference SelectBuilder
expr: raise error when 0-len slice passed
Merge pull request #26 from nickvanw/nvw-empty-eq-slice

expr: raise error when 0-len slice passed
add support for return id in postgres
Merge pull request #27 from rande/add_query_support_for_insert

Add query methods for InsertBuilder
Added support for subquery's alias syntax.

In mysql, subquery should be have specify alias.
Merge branch 'patch-1'