[GH-348] [DDC-1819][WIP] Arbitrary object hydrator

This issue is created automatically through a Github pull request on behalf of marijn:

Url: https://github.com/doctrine/doctrine2/pull/348

Message:

Initial work (in progress) on a test suite for the arbitrary object hydrator, as discussed in DDC-1819[1]. Any tips are appreciated. I'm not too sure what the test suite should and should not cover.

Other questions I have include:

1. Should the `HYDRATE_ARBITRARY_OBJECT` constant be added to the `AbstractQuery` class or the `NativeQuery` class? It only makes sense in the former but it might be missed when more constants are added in the future...
2. Should I use data providers in my tests for the result set data?
3. Should my tests be added to a `DDC1819` namespace?
4. Should I add functional tests?

[1]: http://www.doctrine-project.org/jira/browse/DDC-1819