Creation of the object for calculating interest, etc.
Merge remote-tracking branch 'origin/updates_portfolio' into updates_portfolio

Conflicts:
	asset.rb
	portfolio.rb
irr
Merge pull request #27 from merkhofer/interest_rate_calculators

Internal Rate of Return, NPV takes an array
Merge pull request #28 from skunath/interest_rate_calculators

Interest rate calculators