Add slot.get api tags and comments

Assets on hold.
Introduce and switch to === for models

This provides better run-time type-checking.  Unfortunately, == is still necessary for case matches and equals is still necessary for groupBy and recursive equals (though I'm not even sure if those are using that overload.  Probably the are not.
Correct the remaining equals functions
Demote SlotAsset to AbstractSlot

Add some hacks to get back real slots as necessary.  So far this has no
effect as all SlotAssets are constructed with real slots, but the next
step is to generalize SlotAsset functions.
Add basic assets list to slot.get api

Should consider which segments to include and where, and whether to use inContext.
Merge remote-tracking branch 'remotes/origin/master' into angular

Conflicts:
	conf/routes
Fix old slot_asset queries calls
Merge remote-tracking branch 'remotes/origin/master' into angular
Fix excerpt type change fall-out
Merge remote-tracking branch 'remotes/origin/master' into angular
preliminary models; improved routes; bug fixes in frame and portrait; better volume overview