Add FXMacroData market data source

[roberttidball]

Summary

• Add fxmacrodata to AlphaPy's market data dispatch table.
• Fetch daily FX reference rates from FXMacroData and normalize them into AlphaPy's OHLCV market-data shape.
• Add mocked unit coverage for request construction and dataframe normalization.

Validation

• python -m py_compile alphapy\\data.py tests\\test_fxmacrodata_data.py
• python -m pytest tests\\test_fxmacrodata_data.py was not runnable in my local checkout because iexfinance is not installed; the test itself uses mocked FXMacroData responses.