Support for parsing Comments

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" ]
      }
    ]
  }
}
Add parsing comment codes into bits/70_xlsx.js

Update test xlsx file to apachepoi_SimpleWithComments.xlsx
Comments maybe contain an empty text tag
Share the code for parsing the rich text and use it to parse the comments.
Update the range of the sheet when creating a cell for the content of its comments
Merge pull request #43 from nerds-odd-e/comment_support

Support for parsing Comments