install eos in MacOS

旧版本安装老文：https://www.jianshu.com/p/f26ee4cf1d4a

2018.1 3日更新：现在官方提供脚本，把该文的过程自动化了，
现在安装超简单了：

git clone https://github.com/eosio/eos --recursive

cd eos
./build.sh darwin

[100%] Building CXX object tests/CMakeFiles/chain_test.dir/common/database_fixture.cpp.o
1 warning generated.
[100%] Building CXX object tests/CMakeFiles/chain_test.dir/common/main.cpp.o
3 warnings generated.
[100%] Linking CXX executable chain_test
[100%] Built target chain_test

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