MNN 1.0.0 release sync.

- Added Python Express API implemented with pbind11
- Added demos for Python Express API
- Performance improvements for ARM64, ARMv8.2, x86.
- README update.

add -msse3 to fix _mm_hadd_ps problem when use gcc

Merge pull request alibaba#724 from lanchongyizu/bugfix/_concatbatch

[MNN:Bugfix] fix _concatBatch bug

Merge pull request alibaba#661 from 0ct0cat/master

[MNN:Bugfix] fix complier error on ubuntu 18.04

Merge pull request alibaba#679 from muare/master

[MNN:Bugfix] 修复 Windows x86 backend 编译出错

[Bugfix]fix onnx reduction MAX and MIN bug

Merge pull request alibaba#545 from Guanmoyu/master

[MNN:Bugfix] fix ThreadPool bug

Sync. Fix OpenGL related building issues. Build the whole suite on An…

…droid CI (alibaba#580)

* Sync code with latest internal version

* Update CMake

* Fix logging issues

* Fix OpenGL Building

* Bump CMakeLists version. Update Podspec

* Update MetalLib Lookup logic

* Fix Windows Build

Bug Fix

0.2.1.5

# integration
- add travis CI
- fix building parameters for python

# converter
- add half storage option for MNN converter
- fix op name lost in converter
- fix converter bug for print input output, identity remove output

# ops
- add quantized Convolution & Deconvolution support on OpenCL
- add more expression supports
- add DetectionPostProcess Op for TensorFlow Lite (ssd is supported directly now)
- add supports for LSTM & ELU for ONNX
- add support for Convolution that weights is not constant for ONNX
- fix Unary Op compile error on Linux
- fix Metal backend buffer reuse after resize
- fix Metal raw memory access after model releasing
- fix redundant transpose in Winograd generater