Create an class to work with files affected by Pull

[pnatashap]

For now list of affected files is accessible via JsonObject, it is good to have a separate class to work with it

interface Pull {
...
Iterable<JsonObject> files() throws IOException;
...
}

each file have a structure

{
    "sha": "bbcd538c8e72b8c175046e27cc8f907076331401",
    "filename": "file1.txt",
    "status": "added",
    "additions": 103,
    "deletions": 21,
    "changes": 124,
    "blob_url": "https://github.com/octocat/Hello-World/blob/6dcb09b5b57875f334f61aebed695e2e4193db5e/file1.txt",
    "raw_url": "https://github.com/octocat/Hello-World/raw/6dcb09b5b57875f334f61aebed695e2e4193db5e/file1.txt",
    "contents_url": "https://api.github.com/repos/octocat/Hello-World/contents/file1.txt?ref=6dcb09b5b57875f334f61aebed695e2e4193db5e",
    "patch": "@@ -132,7 +132,7 @@ module Test @@ -1000,7 +1000,7 @@ module Test"
  }