Rename and redirect OperationFeeStats to FeeStats

[PJvGrol]

Although #214 mentioned that the operation_fee_stats endpoint would be replaced by fee_stats and marked it as done, the changes required to the requestbuilder seem to have disappeared in #231

As such the OperationFeeStats didn't work. In this PR the oversight is rectified and OperationFeeStats is replaced by FeeStats, and in particular the request builder now properly uses the /fee_stats endpoint.
It will break if trying to use with a Horizon version < 1.0.0

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.