Move source files to src folder
Replace custom Autoload by the one supplied by composer complient with PSR-0
Add phpunit settings file
Code review on Service abstraction

* Replace specific exceptions (InvalidAppId, NotAllowed, NotFound, Runtime and TooManyRequests) for ResponseErrorException.
* Add ResponseException.
* Remove known error constants.
* Move service request execution logic from fetch() to new method call().
* Add support for service calls to be made with (optionally) a specific set of fields
Add unit tests for Service abstraction
Code review on Rates abstraction

* Remove Historical abstraction.
* Remove Latest abstraction.
* Add UnexpectedValueException.
* Rename getRateByIso4217() to getByIso4217().
* Remove getBaseRate().
* Add $refresh flag to fetch() in order to have the possibility to fetch data only if container is empty or this flag is enforced.
* Update fetch() method, now retrieves the class instance to allow method chainability.
* Move container population logic from fetch() to new method populate().
* Add fetchByIso4217().
Code review on Currencies abstraction

* Replace service properties (appId and secureConnection) as arguments on class constructor by service instance.
* Remove service endpoint class property, which is now exclusively handled by supplied service.
* Rename getCurrencyByIso4217() to getByIso4217().
* Add $refresh flag to fetch() in order to have the possibility to fetch data only if container is empty or this flag is enforced.
* Update fetch() method, now returns the class instance to allow method chainability.
* Move container population logic from fetch() to new method populate().
Merge branch 'dev/service' into develop
Add unit tests for Currencies abstraction
Merge branch 'dev/currencies' into develop
Add tests for Rates abstraction
Merge branch 'dev/rates' into develop
Merge branch 'develop'