adds prototype of capabilities decorator

[willemolding]

• Replaces @zome_function decorator with @capability('capabilityName')
• Correctly creates multiple capability directories and jsons which are detected by the holochain core

Closes #22

[willemolding]

How do we like this as an example? It feels pretty good to me but seems kind of awkward that a single typo can create a whole new capability.

Another idea might be to use namespaces to represent capabilities e.g.

@capability
namespace main {
  function zomeFunction1() {}
  function zomeFunction2() {}
}

This would reduce the number of decorators needed and eliminate the @zome_function one entirely as all functions in a capability namespace would be zome functions