Issue25

Hi,

This is a proposal for issue #25: adding a `toMap()` method to `PropertyMapper` which creates a `Map` representation from its data. I was first considering creating a dedicated `Iterator` to iterate on maps but this solution seems simpler.

Here are the main points:
* I've updated some Gradle syntax (I'm using 2.0).
* I've also update OpenCSV dependency.
* The project now uses Groovy 1.8.8 (1.8.7 and above is required for `collectEntries()` on `Iterable`).
* I've a test in `CsvParserSpec` based on the `toString` test.

Thanks.