Refactoring SDK's code generation patterns

[jasdel]

Updates the SDK's code generation pattern with several improvements to improve discoverability, and consistency in client naming.

• Standardizing of the service model's ServiceID for the name of the
  service client's package.

• Adds an operation Response type that is in addition from the
  operation's Output parameters. This prevents collisions between SDK
  response metadata, marshaling methods, and modeled output parameters.

• Refactor service client package's client name to be named Client
  instead of the service's short name. Removes the stuttering of client
  package and type, (e.g. s3.S3 becomes s3.Client)

• Fix service endpoint lookups use the the service's modeled EndpointID
  instead of inconsistent service name.

• Generate API operations into their own file instead of all operations
  and parameter types being generated into a single file. This improves
  readability of individual source files for documentation reference.
  Non-input/output types are still generated into a single file. This change also fixes several occurrences of incorrectly generated API operation input/output type names.

• Removes aws/endpoints Service identifiers from endpoint list. These
  were not actually service identifiers, but EndpointsID. In addition the
  values modeled by the services were unstable, and could change causing
  breaking changes in the SDK's aws/endpoints package. The service
  client package's EndpointsID const should be used instead.

• Move Paginate method from the initialized Request type into its own
  construct via the NewPaginator function to create the
  request paginator. (e.g. req.Paginate becomes NewListObjectsPaginator(req))

Fix #294 #282 #275

[jasdel]

Notes:

• api_parameter_types.go should probably renamed to just api_types.go
• Paginate's CurrentPage should return the Response type instead of the Output type. This allows access to the request metadata. Not yet possible to return the Response type until SDK's internal request type is refactored.
• Verify Paginator example Next takes context. And add package names to example.

[stealthycoin]

Looks good.