Posts: Add posts Redux state handlers

[aduth]

Related: #708, #2167
Prerequisite of: #303

This pull request seeks to add new basic posts querying handlers to the global Redux state. This will be used in the post editor in enabling a user to search for existing content to link.

Implementation notes:

A few choices worth scrutinized review:

• I chose to index posts by global_ID, so that the items map could be a simple flattened object. Since post IDs are unique to a specific site, global_ID enables us to keep all known posts more readily accessible.
• "Request" as a chosen verb instead of "Fetch". There was some previous discussion in which a concern was raised over using fetch in action names, as network requests aren't always fetch-related.
• Multiple queries can be tracked simultaneously, and are referenced by its JSON serialized query (where a query is the object passed to wpcom.js postsList method). Aside from allowing multiple post queries to be performed, this also prevents unnecessary re-requests when switching back to a previously known query result.

Testing instructions:

Confirm that Mocha tests pass by running make run in the project root directory, or directly from client/state/posts.

The included actions are not currently used anywhere in the application, though you should confirm that the inclusion of the posts reducer does not cause any errors to occur when loading any Calypso page.

[dmsnell]

this is a great example test file and is worth publishing in a weekly digest

[aduth]

this is a great example test file and is worth publishing in a weekly digest

Thanks @dmsnell !

Related, I had struggled for a while to find a way to test the dispatch callback without relying on the return value being a promise. I think this is reflected by the requested Sinon feature at sinonjs/sinon#118, as it could allow me to write something like:

// Before:

requestSitePosts( 77203074 )( spy ).then( () => {
    expect( spy ).to.have.been.calledWith( {
        type: POSTS_REQUEST_FAILURE,
        siteId: 77203074,
        query: {},
        error: sinon.match( { message: 'User cannot access this private blog.' } )
    } );
    done();
} ).catch( done );

// After:

const stub = sinon.stub().calledWith( {
    type: POSTS_REQUEST_FAILURE,
    siteId: 77203074,
    query: {},
    error: sinon.match( { message: 'User cannot access this private blog.' } )
} ).calls( done );

requestSitePosts( 77203074 )( stub );

Curious if anyone has encountered a similar need.

[dmsnell]

I'm just getting started in my journey to learn what sinon offers, but I'm really impressed with it. We could probably all benefit by spending some focussed time reading through their (pretty incredible) documentation.

as far as your response goes - I haven't seen many people besides you actually stubbing up calls like these, so I'm not familiar with other attempts at it.