Lambda provider abstraction

[rogelio-o]

The requests will be created from a mapper which maps the provider events to the framework requests. We can create a mapper for any serverless provider (AWS, Google Cloud, Azure,...) and the framework will support each one, the user only has to tell the app which one has to use. Also, the mapper has to transform the framework response to a valid HTTP response in the provider.

The mapper will be a class implementing the IProviderMapper interface.

The AWS Lambda and Google Cloud mapper will be implemented in the framework lib. More mappers for other providers can be implemented in others external libraries.

The README, project description, docs, etc. have to be changed in order to reflect this new feature. It is important to remove that the framework is only valid for AWS Lambda.

[rogelio-o]

Instead of mappers, the app handle method will recibe abstract objects for input (RawEvent) and output (RawCallback). Transformers will be implemented for each provider we want, to transform from provider event to RawEvent and from provider callback mechanism to RawCallback.

The implementation of AWS and Google Cloud will be split in other libs (lambda-framework-aws and lambda-framework-gcloud). This is because we need the framework as light as possible to be uploaded to out serverless provider without overcome the limit.

The DefaultTemplateLoader also will be moved to the AWS lib and will be renamed to S3TemplateLoader. Another template loader will be implemented for Google Cloud Storage.