Add Omni Currency Example

[whitj00]

Add Omni/Mastercoin asset implementation

[jorisbontje]

This example has a few obvious security flaws:

1. by using the same namespace for storing the account balances as the issuer address.
2. using an integer overflow on value you can steal value from someone else' account using send.
3. revoke is probably lacking a check if the caller is the issuer. (also it takes formvalue from the provided acct, but then deducts it from msg.sender account.