Add support for DEPOSIT-US node type

[dchang]

Trying to retrieve a DEPOSIT-US node via user.find_node(id: node_id) raises a KeyError: key not found: "DEPOSIT-US" error.

Looks like this would involve adding a new DepositUsNode class in line with the pattern surrounding the other node classes.

Also, changing:
https://github.com/synapsepay/SynapsePayRest-Ruby/blob/04f48e69f63c8a27422994f516d0a192bc543f9a/lib/synapse_pay_rest/models/node/node.rb#L95
to:

klass = NODE_TYPES_TO_CLASSES.[response['type']] || BaseNode 

should work in the meantime. Currently, fetch raises the error never reaching the default class.