Corrected Porter's description to be API-agnostic. (#745)

* Added Porter to Third Party APIs section.

* Moved Porter to Data Structure and Storage section.

* Corrected Porter's description to be API-agnostic.

README.md

 * [Knapsack](https://github.com/DusanKasan/Knapsack) - Collection library inspired by Clojure's sequences.
 * [PHP Collections](https://github.com/schmittjoh/php-collection) - A simple collections library.
 * [PINQ](https://github.com/TimeToogo/Pinq) - A PHP library based on .NET's LINQ (Language Integrated Query).
-* [Porter](https://github.com/ScriptFUSION/Porter) - Data import abstraction framework for accessing APIs.
+* [Porter](https://github.com/ScriptFUSION/Porter) - Data import abstraction framework.
 * [Serializer](https://github.com/schmittjoh/serializer) - A library for serialising and de-serialising data.
 * [Totem](https://github.com/Wisembly/Totem) - A library to manage and create data changesets.
 * [YaLinqo](https://github.com/Athari/YaLinqo) - Yet Another LINQ to Objects for PHP.