Handle multiple deposits to gateway address in GatewayJS

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Description

GatewayJS can't currently handle multiple transfers to a gateway address, and it does not warn the user of this either.

Design

Possible solutions include:

1. The Gateway widget lets the user submit multiple signatures to Ethereum
2. The Gateway widget forks into multiple widgets to handle the deposits
3. The user is warned that additional deposits can't be recovered

Multiple solutions could be provided, allowing the 3rd party developers to decide how multiple deposits should be handled.

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GatewayJS will now only be receiving security updates.