Add Collections

[raybesiga]

The client does not fully support the Collections API. We need to add a module to support the following operations:

• POST /token (we need to already have an authorizer for this)
• POST /requesttopay
• GET /requesttopay/{referenceId}
• GET /v1_0/account/balance
• GET /v1_0/accountholder/{accountHolderIdType}/{accountHolderId}/active

Reference

• https://momodeveloper.mtn.com/docs/services/collection