Fixes #8. Added metadata flag and disabled metadata for ZMQ conversion, and corresponding tests.
Added CommContent to distinguish Comm message types from other kernel message contents
Added CommWriter to handle sending Comm-related messages
Added license headers to new files
Removed unused universe import
Added CommCallbacks to handle callback execution for Comm events
Added CommStorage type alias into Comm package object
Added CommRegistrar to facilitate registering Comm callbacks
Updated CommCallbacks to provide a CommWriter as an additional argument (easier for responses)
Added CommOpenHandler to trigger open callbacks
Added link and remove methods to CommRegistrar
Added CommMsgHandler to trigger msg callbacks
Refactored CommId to TestCommId in CommOpenHandlerSpec
Added has/retrieve methods, refactored test contants to have correct casing
Updated CommOpenHandler to link when an open message is received
Added CommCloseHandler to trigger close callbacks
Updated CommMsgHandler to take an unused CommRegistrar (to make consistent with other Comm handlers)
Added docs for CommOpen/CommClose handlers
Refactored ActorLoader to follow 80 character line limit
Added Comm-related properties to bootstrap (handlers/storage/registrar)