Support for parsing Comments

Thanks for a great library!
Here is my best attempt at contributing support to parse comments.
I used an existing file already in the project for the regressions tests.
I realize I coded the changes directly in the built xlsx.
Let me know if you want me to change something.

Thanks!

Comments parts listed in the [Content Types] are parsed.
Sheets's relationships are parsed.
Comments parts are correlated to their corresponding sheets parts.
Comments's contents are added to the ref'ed cells.
Rich text styling properties are currently ignored.

For example:
```
{
  "!ref": "A1:B3",
  "A1": {
    "v": 1,
    "t": "n"
  },
  "B1": {
    "v": "one",
    "t": "s",
    "r": "one",
    "c": [
      { "a": "Yegor Kozlov",
       "t": [ "Yegor Kozlov:",
              "\r\nfirst cell" ]
      }
    ]
  }
}
```