Support for the Transaction Split API

This release of the Paystack API library supports the entire transaction split API surface. The operations include:

Create Transaction Split
List/Search Transaction Splits
Fetch Transaction Split
Update Transaction Split
Add/Update Transaction Split Subaccount
Remove Subaccount from Transaction Split

What's Changed

• Changed control flow for error handling of the client to match statement by @morukele in #7
• added get and post functions to reduce repetition by @morukele in #8

Full Changelog: v0.1.4...v0.1.6