Allow hash of columns as well as an array

allows table definition like this:
var table = sql.define({
  name: 'blah',
  columns: {
    id: { primaryKey: true, type: 'serial' },
    email: { type: 'text' }
  }
});
Auto-join based on column information

for usage please see test/dialects/join-to-tests.js
Merge pull request #68 from brianc/auto-join

Auto join