cross version docker tests

[Walter Schulze]

I have finally figured out how to make tests with docker.

Now I can test all combinations of protoc versions 2.3.0, 2.4.1 and 2.5.0 with go versions 1.2.2 and 1.3

All the code can be found in the testdocker folder.
By typing `make run` all the Dockerfiles with be generated, the dockers will be built and the tests will be run.

The Dockerfiles are generated using a small go program called gogodocker-gen which reads a json config, see config.json in the the gogodocker-gen folder.
In this folder you will also find the Dockerfile template from which the others are generated.

I think this is quite a useful technique to guarantee support for specific versions of your dependencies.