Adds .podspec file for use with Cocoapods dependency manager for Objective-C projects

CocoaPods is a dependency management system for Objective-C projects.
This pull request contains a podspec file to allow iOS users to install and use PGMidi through CocoaPods.

- Lint of podspec 'WARN's when analyzing due to lack of tag to point at in 's.source', currently pointing to ':commit => "a5c91d489b54f5ac421c12e0243a236812ab1021"' as a temporary work around until main repository has proper tags for releases
- Lint of podspec 'WARN's when analyzing due to lack of a license file in project