feature request: update notification

[Costia]

something that will nag the users to update when a new version is out.
no idea how to implement
presumably put some kind of a tag in the block chain

this can also lead to different types of validation in the future:
 i.e blocks 1-100 were generated by version 0.2 - they should be validated in a certain way, blocks 100-end created by the current version, no special treatment is needed