executeAndFetch fallback to executeScalar

This is a second try at https://github.com/aaberg/sql2o/pull/45.

*  `executeAndFetch` methods fall back to `executeScalar` methods if:
  * there is only one column returned
  * no property on the given class matches the column name 
  * we have a converter for the class
* `executeScalarList` now uses registered converters, see https://github.com/aaberg/sql2o/issues/36
  * **WARNING** this changes the API because a class now needs to be passed in to use the converter
  * This is good though, the method should have required a class previously