Allows comparisons between extractors and values

``` scala
  monetaryValueExtractor ==== amount

  implicit class ExtractorComparisionOps[A](extractor: sqlest.extractor.Extractor[ResultSet, A]) {
    def ====(value: A): Column[Boolean] =
      extractor.settersFor(value).map {
        case Setter(column, value) => sqlest.ast.InfixFunctionColumn[Boolean]("=", column, value)
      }.reduce(_ && _)
  }
```

This would fix #65 question 2
