Allow plugins to change time tracking validation

Firstly, this is my virgin JIRA JIRA, so please excuse any errors I've made.

Petur Agustsson of TM Software, makers of Tempo, would like to be able to control the way JIRA validates time input. He writes:

JIRA is currently validating work logs. It checks if work logs are enabled and also makes sure that the user has all the correct privileges to enter the work log (and possibly something more). We would really like to see some way for plugin vendors to extend those checks since there are many ways to enter work logs and Tempo is just one way. If we could extend the standard JIRA checks, every work log would have to pass the same validation regardless of the plugin used to enter the work log. This would ensure the accuracy of the data which is of course crucial.