Save ticks lived to nbttag

Since this attribute is about how long an entity has lived (and not about how long ago was the last server restart *g*), the information should be stored.

**Usage example:**
We're using this information for example to know the real age of a animal to add some "real feel" to animal aging and to add the feature to let animals die after some time to create the need to care about them and create baby animals ;-)
This adds some tiny cool idea while finally removing huge abandoned farms.
Example plugin: https://github.com/Cube-Nation/MobAge