feat: Added 'statusCode' to RpcException

Provides immediate access to the most important part of the exception
feat: 'range' optional argument added to downloadObject

Was available in previous versions, but removed
feat: Fixed fs library
feat: Added testing library

Exposes a MockConnection object which stores remote files in memory
test: Added filesystem tests (which pass)
chore: Bumped version to 0.3.1-pre1