Add a basic-auth-based client for WP-API usage

Requires a blog url, username, and password.
WP-API supports other HTTP methods like PUT

Mark those are valid methods.
WP-API uses 201 as a success response

Add that as a valid response code.
Rename WP_API to WPAPI for consistency
Rename non-WPCOM-specific class prefix to WP

Since we can use them for both WPCOM and WP-API.
Don't require WP_HTTP_API Transport by default

Since we don't actually use it
Add loader files for wpcom and wpapi

To make it easier to load the API clients.

We should find a cleaner way to load and organize files.
WP-API client should return the response

Otherwise we end up with a null
Rename WPCOM_REST_Exception with WP prefix

Since it's currently a generic class
Fix require path for renamed Exception class
Only build http query if we have params

Fixes warnings thrown when build_url is passed a null for the params
Introduce WPAPI_REST_Object_Post

For interacting with individual post objects via WP-API
WP-API suggests using PUT to update objects

Instead of POST
Brevity for update and delete methods

Just call them `update` and `delete`; no need to suffix with `_post`
Prefix factory methods for objects with init

Clearer that these methods are used to initialize the class and sounds
better to param-less object instantiation like WPCOM's Me and WPAPI
Posts.
New class for fetching Posts Collections

And related objects like post types and statuses.
Readme: WP_HTTP_API usage needs a require
Readme: add note about exceptions and errors

Should be handled for all API requests
Readme: WP-API usage instructions
Merge pull request #2 from Automattic/feature/wp-api

Add support for WP-API