Add profile methods/giftcard issuers/voucher issuers and settlements

[Pimm]

This PR adds some binders:

• profileMethods (closes #253)
• profileGiftcardIssuers
• profileVoucherIssuers
• settlements (closes #270)
• settlementPayments
• settlementCaptures
• settlementRefunds
• settlementChargebacks

It also changes the NetworkMocker class to have a simpler interface.

The added tests are not executed on Node.js 6.14 (see jest.config.js). However, support for Node.js < 8 will be dropped in the next major release anyway.

Solves #253 and #270.