Allow setting device id by -gpu.

[walkerning]

Hi, we are using your repo. At first, it's confusing when we find setting device_id and solver_mode in dqn_solver.prototxt does not work. There are inconsistency with the API of the official caffe tool.

I think the inconsistency with the official caffe tool does not matter, however, I think: at least it's necessary for me to be able to set device_id by command line arguments like this:

./dqn -gpu 2

this is a little patch.

btw, Thank you for your work.