Add tf GPU options

[ryanjulian]

It should be possible to set the TensorFlow session options whenever a tf.Session is created for training, such as here (other places where a session is constructed might not need to use these options). It is sometimes necessary to limit the available memory to tf running on a GPU, etc.
A possible implementation could allow the user to specify the GPU options via a ConfigProto setting in config_personal.py (set to None by default).

Imported from ryanjulian/rllab#123

[ryanjulian]

This is more of an experiment runner concern, actually.