NOJIRA : Adding a Spreadsheet XSLX export with media icons included

Exchanges with Julia :
https://github.com/collectiveaccess/providence/pull/66#issuecomment-31689743

Modifications :
- Defaulting columns width when text
- Adding media icon as first column if no media representation is in the result table
- Calculating columns width & rows height from media version (no, auto size & height don't work with images in phpexcel, grumplf)
- 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
- image offset from top & left to center icon in its cell when the icon version is the only one
(cherry picked from commit 0992927)