Enable optimizer model setup with instantiation

[Crissman]

Code to close issue #3211 .

Allow models to be passed as an argument to an optimizer, so

optimizer = chainer.optimizers.SGD()
optimizer.setup(model)

becomes:

optimizer = chainer.optimizers.SGD(link=model)

Change added to all optimizers. Old two-line method also still valid.

Tested by running new one-line method and old method on SGD, then running new method on all optimizers after changing code.

[mitmul]

@Crissman Check the travis-ci's test errors and please fix them.

[mitmul]

Jenkins, test this please

[mitmul]

LGTM

[niboshi]

Note that this code does not work, because the first argument of SGD is lr.

optimizer = chainer.optimizers.SGD(model)

I think it's better to make model a keyword-only argument.

[Crissman]

@niboshi Correct. model should be a keyword argument. I've updated my comment above accordingly.

[kmaehashi]

Removed to-be-backported (as discussed in #3936)