added ValuationContext

A ValuationContext has environment, refdata and valuation params.

OptionOnFutureValuation implemented.
added very basic swap and future valuation

also key recording environment. valuation is pretty simple at the moment. mostly
 just an attempt to figure out the structure.
Fleshed out some swap code

Added Spot prices. Swaps on Spot Markets
Changed curves so they return Either rather than throw an exception
 if there's missing data.
Merge pull request #8 from og3b/master

added ValuationContext