Fix collisions with vanilla DerpKit

Without making these changes, you can't include OvershareKit in
an app that already uses DerpKit.
Merge pull request #3 from berg/derpkit-fix

Fix collisions with vanilla DerpKit
Add OvershareKit podspec
Fix authentication bugs specific to Facebook.
Merge pull request #2 from jhersh/master

Add OvershareKit podspec
Update podspec.
podspec tweaks, fix warning
Merge pull request #7 from jhersh/master

Podspec tweaks/warning fix
Fix content size calculation bug when setting text via setText:
Fix color bug. Change App.net web flow redirect URI to the default 'http://localhost:8000' used by ADN.
Update AirDrop compatibility utility.
Update podspec.
Increase image upload quality for smaller images and iOS device screenshots.
Automatically shorten long URLs when creating microblog posts.
Refactor session management to use OSKSessions to keep track of session identifiers and completion handlers. Rename OSKFlowController to OSKSessionController. Add an option to skip the activity sheet altogether and begin a sharing session with a given activity (so you can use your own share sheets).
Add comment.
Update sample app to use OSKSessions. Fix sign in bug with Instapaper. Fix iPad bug that prevented presentation ending handlers from being called under certain conditions on iPad.
Expand comment.
Fix crasher: not checking for nil before invoking an ending handler.
Merge branch 'dev' of github.com:overshare/overshare-kit