Added JSON schema for Evidence release

Added main Release JSON schema for Evidence.
Added included JSON schema files.

spec/fixtures/api/schemas/evidence/author.json

+{
+  "type": "object",
+  "required": [
+    "id",
+    "name",
+    "email"
+  ],
+  "properties": {
+    "id": { "type": "integer" },
+    "name": { "type": "string" },
+    "email": { "type": "string" }
+  }
+}

spec/fixtures/api/schemas/evidence/issue.json

+{
+  "type": "object",
+  "required": [
+    "id",
+    "title",
+    "description",
+    "author",
+    "state",
+    "confidential",
+    "created_at",
+    "due_date"
+  ],
+  "properties": {
+    "id": { "type": "integer" },
+    "title": { "type": "string" },
+    "description": { "type": "string" },
+    "author": { "$ref": "author.json" },
+    "state": { "type": "string" },
+    "confidential": { "type": "boolean" },
+    "created_at": { "type": "date" },
+    "due_date": { "type": "date" }
+  }
+}

spec/fixtures/api/schemas/evidence/milestone.json

+{
+  "type": "object",
+  "required": [
+    "id",
+    "title",
+    "description",
+    "state",
+    "created_at",
+    "due_date",
+    "issues"
+  ],
+  "properties": {
+    "id": { "type": "integer" },
+    "title": { "type": "string" },
+    "description": { "type": "string" },
+    "state": { "type": "string" },
+    "created_at": { "type": "date" },
+    "due_date": { "type": "date" },
+    "issues": {
+      "type": "array",
+      "items": { "$ref": "issue.json" }
+    }
+  }
+}

spec/fixtures/api/schemas/evidence/project.json

+{
+  "type": "object",
+  "required": [
+    "id",
+    "name",
+    "description",
+    "created_at"
+  ],
+  "properties": {
+    "id": { "type": "integer" },
+    "name": { "type": "string" },
+    "description": { "type": "string" },
+    "created_at": { "type": "date" }
+  }
+}

spec/fixtures/api/schemas/evidence/release.json

+{
+  "$schema": "http://json-schema.org/draft-04/schema#",
+  "id": "http://example.com/release.schema.json",
+  "type": "object",
+  "required": [
+    "id",
+    "tag",
+    "name",
+    "description",
+    "created_at",
+    "project",
+    "milestones"
+  ],
+  "properties": {
+    "id": { "type": "integer" },
+    "tag": { "type": "string" },
+    "name": { "type": "string" },
+    "description": { "type": "string" },
+    "created_at": { "type": "date" },
+    "project": { "$ref": "project.json" },
+    "milestones": {
+      "type": "array",
+      "items": { "$ref": "milestone.json" }
+    }
+  }
+}