Update to new xapi-storage.0.2.1

This follows the API changes in xapi-storage.

New message-switch API

Merge pull request #13 from djs55/message-switch.0.11.0

Update to the new message-switch API

Interface updates

• Update to SMAPIv2 with o_direct, o_direct_reason
• Update to using json marshalling for exception backtraces
• Use modern travis configuration
• Add opam file for development

Minor fixes

• Treat VDI.epoch_{begin,end} as no-ops
• Write the pidfile when daemonizing

Initial prototype

• daemonization works
• syslog works
• basic operation seems to work