Adding Cocoapods support

Adding a podspec file for using UsefulBits as a cocoa pod. At present, I've added:

    pod 'UsefulBits', :git => 'https://github.com/mattconnolly/Useful-Bits.git'

To my podfile. I'm working on an iOS project, so I haven't tested this yet for OSX, but it should be okay.

There's already a "1.0" tag in your repo, which this podspec will reference. It seems a requirement to have a tag match the version in the podspec file.