Traceability release

• add profiling/tracing support
• add a "Front.wait_for_free" function to wait for n free slots
• add opam file

Interface now supports xenstore ring reconnection

In order to reconnect to an existing (xenstore) ring after a restart we must avoid (i) removing data which we haven't persisted somewhere; and (ii) avoid writing the same chunk of data twice. We make this possible by exposing a 'type position' and using this in both 'read' and 'write'. The position represents the current offset in the data stream. The position is manually advanced by the client via the 'advance' function.

Supports Mirage 1.0

Merge pull request #7 from avsm/master

bump to v1.0.0