Contracts infrastructure.

• EToken2.sol acts as a base for all assets(tokens) operation (like issuing, balance storage, transfer).
• Asset.sol implements specific asset properties (described in AssetInterface.sol) and can be extended.
• AssetProxy.sol acts as an asset entry point, provides an ERC20 interface (described in ERC20Interface.sol) and forwards calls to asset implementation. Also allows for asset implementaion upgrades.
• EventsHistory.sol holds all operations events to avoid events lost in case of base contract replacement during upgrade or extension.
• EToken2Emitter.sol provides EToken2 events definition.