Provide error and ready events

[bergie]

This makes defaultGraph initialization asynchronous so that callers can handle its results via events.

Also adds events to the base runtime class:

• ready: if defaultGraph is supplied, emitted when that network is ready. Otherwise emitted after runtime is ready
• error: if network can't be created for the defaultGraph

Fixes noflo/noflo#603

[coveralls]

Coverage increased (+1.6%) to 41.709% when pulling 751fb36 on error_port_test into 07a4954 on master.