Add default currency for a portfolio
Calculate asset value for portfolio
Refactored tests in preparation for data changes

- Renamed Quotes -> RawQuotes
- Renamed Historical -> RawHistorical
- Start and End dates are expressed as time.Time
- # of records is checked
- NOTE : content of records is still not checked
Quotes becomes RawQuotes, Quotes will parse records internally
Refactored historical data

- Historical -> RawHistorical
- Use time.Time instead of custom Date format
- Add debug() function
- Return proper error
- Fix bug: in DEBUG, function would return empty records
Value of Stock assets: use historical data (yay!)

TBD: conversion between currencies