[api] Asset manager

[MitchPierias]

Simplify the deployment of eosio system contracts like eosio.token, and provide api tools to easily interact with them where needed.

• API should automatically deploy eosio.token and allocate system assets in development mode? Attaching the account to an easily accessible method on AccountManager?
• Implement createAssets and issueAssets on the Contract class.
• Create a System class to manage the eosio.token contract and others.
• Provide a System.getAsset('SYMBOL') method to fetch the table rows for the specified asset symbol.
• Provide a System.getAssets('accountname') to get the accounts holdings.

This could all be better by switching System to an AssetManager, and simply manage an asset symbol along with wrapped eosio.token calls.