* Add mips architecture optimization support

* Add mipsisa32r6 toolchain files

* Add AbsVal_mips layer

* Remove unused constructor override

* Keep the same programming style with upstream

* Add Bias_mips layer

* Add Clip_mips layer

* Update copyright header

* Add mips-mti-gnu compile to travis

* Add codescape mips toolchain config

* Update the toolchain comments

* Fix conflicts in travis config file

* Add travis status in README.md

* add relative README_CN.md;
* obtain time cost with op->forward().

* adding how to build for jetson

* update build instruction for NVIDIA Jetson

The latest CMake generates lots of the same warning message as below,
when building ToT ncnn for iOS:

  CMake Warning (dev) at toolchains/ios.toolchain.cmake:451 (set):
    implicitly converting 'string' to 'STRING' type.

This PR fixes 'string' to 'STRING' to suppress the warning messages above.