update readme.txt
Javascript error if #picker-$id is undefined

Case:
You have two different taxonomy:
* one with a meta color type
* a other with a meta image type

When you want to add an image to a term, you will have the error :
"Cannot read property 'farbtastic' of undefined"

To prevent that error we must first check if our id exist in that page.
Get the right image size and don't deform it
Avoid a notice in debug mode when $_GET['tag_ID'] is not defined
Merge pull request #13 from ArnaudBan/master

Avoid a few errors and improve image meta