Okama

Documentation added for all methods and attributes in Assets, AssetList and Portfolio.

Several important updates in Portfolio and AssetList classes:
- Portfolio is an asset (like stocks, ETF or commodities) and can be included in the AssetList
- Portfolio has a symbol (.PF namespace)
- Rebalancing period is an attribute of Portfolio
- Asset has `.close` and `.adj_close` properties (close time series)
- Dividend time series are adjusted to the base currency in Portfolio and AssetList
- `.dividend_yield` for Portfolio is a weighted sum of the assets dividend yields (adjusted to the base currency)
- Portfolio has `.dividends` method (dividends history time series)
- New notebook in examples for investment portfolios (examples/03 investment portfolios.ipynb)

Several improvements to Portoflio and AssetList classes:
- `get_return_ts` has rebalancing_period parameter
- `get_rebalanced_portfolio_return_ts` method in Portfolio is depreciated
- `get_rolling_cagr` and `get_rolling_cumulative_return` could use `real=True` option to calculate real return values
- `get_cagr` and `cumulative_return` have `real` option
- Several new docstrings in Portfolio and AssetLists classes

Project RoadMap and Contributing guidelines are added to GitHub repository.

Several bugs are fixed.
New methods in Portfolio class:
- get_cagr (instead of cagr property)
- get_cumulative_return
- get_rolling_cumulative_return

setuptools builds correctly the package after updating setup.py.