Updated lotus-go client

[vaddenz]

Updated lotus-go client with integration tests (test coverage: 81.8%), see README for details.

Currently Supported Methods:

1. Tracking Events
   • Track Event
   • Verify Event Ingestion
2. Customers
   • List Customers
   • Get Customer
   • Create Customer
3. Credits
   • List Credits
   • Create Credit
   • Update Credit
   • Void Credit
4. Subscriptions
   • List Subscriptions
   • Create Subscription
   • Cancel Subscription
   • Switch a Subscription's plan
   • Update Subscription
5. Access Management
   • Get Feature Access
   • Get Metric Access
6. Plans
   • List Plans
   • Get Plan
7. Add-ons
   • Attach Add-on
   • Cancel Add-on
8. Invoices
   • List Invoices
   • Get Invoice
   • Get Invoice PDF Url

[vaddenz]

@mnida Hi, please take a look when possible :D The SDK is ok for first release.