Created 获取oss服务授权接口 (markdown)

+####获取oss授权接口
+接口地址：/osspolicy
+请求方式：GET
+参数：
+|参数         |是否必须         |说明               |
+|-------------|---------------|------------------|
+|type         |否             |类型               |
+|key          |否             |图片的相对路径。key不为空，会返回preview(图片的绝对路径) |
+
+返回值：
+
+**key值不为空**
+```json
+{
+  "preview": "http://avatars.boxfish.cn/abc.jpg",
+  "service": "oss",
+  "oss": {
+    "bucket": "boxfish-avatars",
+    "expiration": "2016-01-01T12:00:00.000Z",
+    "region": "oss-cn-beijing.aliyuncs.com",
+    "policy": "eyJleHBpcmF0aW9uIjoiMjAxNi0wMS0wMVQxMjowMDowMC4wMDBaIiwiY29uZGl0aW9ucyI6W3siYnVja2V0IjoiYm94ZmlzaC1hdmF0YXJzIn0sWyJjb250ZW50LWxlbmd0aC1yYW5nZSIsMCw1MjQyODgwXV19Cg=="
+  }
+}
+```
+
+**key值为空**
+
+```json
+{
+  "service": "oss",
+  "oss": {
+    "bucket": "boxfish-avatars",
+    "expiration": "2016-01-01T12:00:00.000Z",
+    "region": "oss-cn-beijing.aliyuncs.com",
+    "policy": "eyJleHBpcmF0aW9uIjoiMjAxNi0wMS0wMVQxMjowMDowMC4wMDBaIiwiY29uZGl0aW9ucyI6W3siYnVja2V0IjoiYm94ZmlzaC1hdmF0YXJzIn0sWyJjb250ZW50LWxlbmd0aC1yYW5nZSIsMCw1MjQyODgwXV19Cg=="
+  }
+}
+```