Make Net master of its phase and device

[shelhamer]

Leaving phase and device to the Caffe singleton is awkward and spells trouble for parallelization and device abstraction.

Net should own phase and device, which are to be set at instantiation, so that nets can co-exist without the delicate coordination of phase and device for parallel execution within and across nets.

• give Phase to Net Give phase to Net #1790
• give mode / device to Net
• diffuse mode / device to Layer

At the same time phase issues like #192 #1250 #1430 should be settled at the same time. Phase has been settled in #1790.

@longjon @jeffdonahue

[shelhamer]

p.s. pycaffe is still weird for certain configurations since phase and device are set too late. I keep forgetting to push a stop-gap patch but perhaps we should fix this all at once and soon. Fix in #1728.