Calculate characters remaining when attaching image

[Cassidy James Blaede]

Right now it's possible for me to attempt to send a 140-character tweet when I've attached an image. Birdie then silently fails, not sending the tweet even though it looks like it sends. Instead, Birdie should calculate the number of characters remaining when I attach an image (140 minus what I've typed minus the length of the image link).

One possible way of doing this is to start uploading the image right when I attach it and replace the characters remaining with a spinner signaling that it's being calculated. As soon as the image URL comes back, subtract its length from the remaining character length.