FetchResult#toPromise

[jhollingworth]

For cases when you want to consume a fetch result outside of a component we want to be able to transform a FetchResult into a promise. FetchResults know their Id and what store they came from but stores won't tell us when a fetch changes (only when the store state changes). To get around this I suggest we add Store#addFetchChangedListener() which calls any listeners just before a fetch result is returned.

var fetch = require('marty/fetch');
fetch.done({foo: 'bar'}).toPromise().then(function (result) {
  console.log('resolved immediately', result); 
})

fetch.failed(new Error()).toPromise().catch(function (error) {
  console.log('failed immediately', error);
})

fetch.pending().toPromise().then(function () {
  console.log('this wont result'); 
})

Resolves #131

To do

• Add Store#addFetchChangedListener()
• toPromise should still work if we don't pass in an Id/store
• Tests
• Docs

[jhollingworth]

You can't change the branch a PR merges into so moving this to #180