Unify the fluid imports into `paddle.v2.fluid`

[reyoung]

The current imports of fluid is quite noisy. We could simplify them by adding imports in fluid/__init__.py

Paddle/python/paddle/v2/fluid/tests/book/test_fit_a_line.py

Lines 2 to 8 in e800c0d

	import paddle.v2 as paddle
	import paddle.v2.fluid.core as core
	import paddle.v2.fluid.framework as framework
	import paddle.v2.fluid.layers as layers
	from paddle.v2.fluid.executor import Executor
	from paddle.v2.fluid.io import save_persistables, load_persistables
	from paddle.v2.fluid.optimizer import SGDOptimizer

The user API could be

import paddle.v2.fluid as fluid

data = fluid.layers.data(...)
loss = fluid.layers.fc(data, ...)
adam = fluid.optimizers.Adam()
adam.minimize(loss)
exe = fluid.Executor(...)
exe.run(feed={...}, fetch=[...])