NOJIRA : Adding a Spreadsheet XSLX export with media icons included

See #66 for more infos and exchanges with Julia

#### Modifications :
- [x] Defaulting columns width when text
- [x] Adding media icon as first column if no media representation is in the result table
- [x] Calculating columns width & rows height from media version (no, auto size & height don't work with images in phpexcel, grumplf)
- [x] Adding a caGetSizeForMediaIcon helper inside mediaPluginHelpers, this returns height & width for a media version from config : I don't want to check each media height & width from file to avoid slowing the process
- [x] image offset from top & left to center icon in its cell when the icon version is the only one