Transaction Charge will only be sent when provided

• Transaction charge has sentinel value of -1 by default
• Rewrite Encoder to ignore negative transaction_charge

Add ChargeCard functionality

• Add Verve cards support

Ensure Keychain Access

• Fix issue where app is unable to get token because we are unable to access the Keychain now that Keychain access requires an entitlement.

Paystack iOS library v1.0.0

Add PSTCKRSA.h to public headers

- Add PSTCKRSA.m to build
- Add PSTCKRSA.h to Paystack.h