build support for functions being assigned to specific capabilities

[Connoropolous]

Willem had an idea for another decorator...

@capability('main') for example

This would then affect the autogen JSON manipulation we're doing, to generate the capabillities/zome JSON

[willemolding]

Its between that and @zome_function('capability_name'). Since every zome function needs to belong to a capability they are kind of the same. I think I prefer the @capability one