Mojaloop extension: Currency conversion

[elnyry-sam-k]

Linked to the Mojaloop specification issue: mojaloop/mojaloop-specification#89

As an Adopter of Mojaloop needing to support cross-currency transactions
I would like to have built-in cross-currencty transactional support
So that I can ensure that

• the integrity of the transfer is always maintained and meet the mojaloop transaction required characterisitics
• that although at lease three parties are involved, only two parties form and agreement of terms, and those terms are not shared with other participaint not part of the agreement
• that foreign exchange providers are catered for as entities providing local collateral