This commit contains an initial prototype for the exporter daemon
that can discovered via and endpoint file on the host.
The exporter looks for the endpoint file to see the availability
of the daemon and exports if it is running.
Exporter daemon will allow OpenCensus users to export
without having to link a vendor-specific exporter in their final
binaries. We are expecting the prototype exporter is going to
be implemented in every language and registered by default.
Updates https://github.com/census-instrumentation/opencensus-specs/issues/72.
JBD