v0.3.1 released

Add snapshots management: create/ delete/ restore/ query
Support insert/eject media in an instance, add media attached icon in status.
Lock volumes when long time running task execute to protect integrity.
Add create and modify time of images and instances.
Add instance control functions in monitor page.
CPU/Memory usages change to bar-chart for more details.
Instance detail and monitor pages open in a new tab by default, suitable to manage multiple instances in parallel.
Fixed: Incorrect instance name when creating a new instance in v0.2.3
Fixed: The running status of the instance in Core is not synchronized if Cell does not notify event correctly in some cases.