Merge pull request #8 from og3b/master

added ValuationContext
MarketDataStore
Post review fixes
Merge pull request #9 from thoozaho/stores

MarketDataStore
 #10 bug in multiplication of UOMs

nasty bug in UOM.asPrimeMap. When adding two maps together it could lose
 data from the second map.

 Fixes #10
RFC - position code