Add _columns method to Struct results.

[lwoodson]

• Currently, the struct results of a query will contain no behavior beyond having attributes matching column names.
• It would be nice to be able to introspect a result struct for the columns/attributes it has.
  • reflective programming
  • tinkering in IRB.
• Add a _columns method to the struct (see http://blog.rubybestpractices.com/posts/rklemme/017-Struct.html) that will return an array of column/attribute symbols in the struct.
• Add an _attributes alias for _columns.