Date cell format

[timotheemoulin]

Hi,
this pull request fixes the format used for date cells.

When retrieving the value, it will be formatted as a MySQL value "Y-m-d H:i:s".

This will allow using the value without having to convert it from the MS Excel time to Unix time.

[gam6itko]

Why have you rewritten the composer project name ? You shouldn't do that! If you want to use your fork in your own projects use repositories key.

"repositories": [
    {
        "type": "git",
        "url": "https://github.com/fcha/spreadsheet-reader.git"
    }
]