Put in checks for dispatch-sync

[mike-thompson-day8]

dispatch-sync can't be called from within an event handler - only an async dispatch can be "called".

Warn if this is ever attempted.

Why is it a problem? Well the the first event handler is given a snapshot of the db as a parameter. And whatever this first event handler returns will be put back into app-db when it finishes. So any changes made by an intermediate call to dispatch-sync will be lost.

So the problem sequence would be:

1. first event handler called with db snapshot
2. dispatch-sync called, making changes to app-db
3. first event handler finishes, and its returned db is written into app-db

Ie. the changes in step 2 are lost. (no longer in app-db)

[mike-thompson-day8]

FAQ entry added:
https://github.com/Day8/re-frame/wiki/FAQ#3-why-cant-i-call-dispatch-sync-in-an-event-handler

[mike-thompson-day8]

New error added. Will be a part of v0.3.2