adding 15.0_High_Availability_and_Scalability.md; first commit; test
updating TOC in 15.0
adding 15.1 through 15.5
adding images for 15.0 to 15.5
Merge remote-tracking branch 'upstream/master'
15.7.0_MySQL_Proxy

第一次提交
删除提交错误的文件
15.7部分章节提交
Merge branch 'master' of https://github.com/xiangp152/manual56