podspec (CocoaPods) file for iOS users

CocoaPods is a dependency management system for Objective-C projects
This pull request contains a podspec file to allow iOS users (and perhaps OS X users too) to install and use libpd.

I have tested it with iOS 5.1 but not with OS X. I believe that it should work under OS X too, unless the compiler flags are different.

Notes:
* Authors -  I wasn't sure how to fill the "authors" field, so I left it empty. It can be easily modified to include libpd authors by following the instructions in [this link](http://docs.cocoapods.org/specification.html#authors).
* Version - was set to 0.0.1, as I wasn't sure if libpd has a version number.

To test it, follow the instructions ("Get started") in [cocoapods.org](http://cocoapods.org) and use the following as the Podfile:

    platform :ios, '5.0'
    pod 'libpd', :podspec => 'https://raw.github.com/kshahar/libpd/master/libpd.podspec'