Additional Backends (Oracle, Stardog, Neo4j)

This Pull Request adds support for the following Triple Stores:

- [Oracle Database](http://www.oracle.com/technetwork/database/options/spatialandgraph/overview/index.html) with Spatial and Graph Option
- [Stardog](http://stardog.com/)
- [Neo4j](http://www.neo4j.org/) with [SPARQL-Plugin](https://github.com/neo4j-contrib/sparql-plugin)

A Connector abstraction layer is introduced to allow adding new backends with minimal effort.
It uses several helper classes to hook into the existing Adapter layer:
![connector-integration](https://cloud.githubusercontent.com/assets/1383323/3492318/760c2cf0-05a7-11e4-939e-d166e5555a21.png)

Generic Connector test cases are provided to simplify adding new Stores.
Additionally, benchmarks (modeled with [Athletic](https://github.com/polyfractal/athletic)) are included. These are useful for optimization and Store comparison.

A [Travis CI](https://travis-ci.org/) configuration was added to run unit tests and MySQL integration tests with several PHP versions. You might want to activate the corresponding WebHook for your repository to keep this integration. If you do not need it, then delete the configuration file `.travis.yml`.

The system relies on the Dependency Manager [Composer](https://getcomposer.org/) to install its used libraries. Therefore, you might want to register Erfurt at [Packagist](https://packagist.org/) to ensure that anybody can install it via Composer without further configuration.