* unify DataLoader APIs, test=develop

* integrate iterable CPU Dataset, test=develop
add GPU dataset supporting, test=develop

* add unittests for dataset, test=develop

* add more docs to dataloader apis, test=develop, test=document_preview

* refine doc, test=develop

* refine doc again, test=develop

* increase coverage, test=develop

The fixed APIs:
6 Methods in paddle.fluid.io.PyReader
paddle.fluid.layers.Preprocessor
paddle.fluid.layers.py_reader
paddle.fluid.io.save_params
paddle.fluid.io.save_persistables

test=develop
test=document_preview

* use pyreader to read data

* add return_list to PyReader to support return value represented as list

* Fix data and reader related api doc

* Fix data and reader related api doc

Review and fix the example code in some reader related API doc.

These APIs are:

Fix existing API example codes:

paddle.fluid.io.PyReader
paddle.fluid.layers.batch
paddle.fluid.layers.data
paddle.fluid.layers.Preprocessor
paddle.fluid.layers.py_reader
paddle.fluid.program_guard

Add new example codes:

paddle.fluid.io.PyReader.decorate_batch_generator
paddle.fluid.io.PyReader.decorate_sample_generator
paddle.fluid.io.PyReader.decorate_sample_list_generator
paddle.fluid.io.PyReader.reset
paddle.fluid.io.PyReader.start

test=develop

* Add changes to API.spec after changing doc.

test=develop

* Add blanks after python example code

test=develop

* Add blank line at py_reader example code

test=develop

* Merge API.spec

test=develop

* Modify reader.py based on reviewer's comment

test=develop

* Modify API.spec after changing doc

test=develop

* Change reader.py based on reviewer's comment

* Modify example code of decorate_sample_generator

test=develop

* Fix example code of PyReader based on reviewer

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop