feat: add a health check handler (#121)

Description

• add a GET /healthz endpoint
• add a version package
• set the app version and build time via ldflags
• add version and build time to response

Related issue

#121 (closed)

Examples

{
  "version":"0.3.1-20200427-0550"
}

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes, specifications reflect those changes and they have been reviewed OR there are no API changes

Closes #121 (closed)