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