opentxs: remove unused variables

Especially, the removal of the AssetAccount string is important
as it conflicts with the upcoming rename of OTAssetAccount into
AssetAccount.
OTAssetContract: rename class to AssetContract
OTAssetContract: rename files to AssetContract.{h,c}pp
Merge pull request #383 from toxeus/renameOTAssetContract

OTAssetContract: rename to AssetContract