Merge pull request #141 from ideal/master

关于编译工具，ngx_str_t变量赋值，ngx_prealloc()的几处修改. Thanks to ideal.
Update chapter_02.rst

修改笔误：”是否“->”释放“
Merge pull request #143 from RocFang/patch-1

Update chapter_02.rst, fix typo, thanks to RocFang.
修改关于nginx core的描述。

See https://github.com/taobao/nginx-book/issues/144