Add example strategies on foreign markets

[enzbus]

Add London / few Europe / Tokyo strategies, same model of current 3 in examples/strategies.

Issues:

• alternative cash keys need testing, improvements in fred data cleaning
• yahoo finance data can be buggy on non-us stocks, needs refactoring and improving data cleaning

Idea:

• factor data.py in data/ submodule;
• inherit YahooFinance from OHLCV class
• refactor data cleaning logic (separate logical steps) in OHLCV, reused with other sources

[coveralls]

coverage: 98.938%. remained the same
when pulling d745b00 on foreign_strategies
into 47084ed on master.

[enzbus]

Replaced by #127.