NDK file support

This PR adds support for the NDK files of the GlobalCMT project.

It includes tests, documentation, and support for Python 2.6-3.4. Ready to merge from my point of view.

It furthermore makes three changes to the event and QuakeML handling:
* Multiple `DataUsed` objects possible per `MomentTensor` objects as required by the QuakeML standard.
* Equality checks for events now actually check the whole event and not only the id.
* Empty optional objects are no longer created when QuakeML is read.