Updated plugin activation and deactivation functions to be multisite compatible.

Check if the plugin is network activated in Multisite, and if so run activation or deactivation function for each public blog in the network.  Initial pass at issue #48.
Added conditional to WP_JSON_Posts->prepare_author() to verify the $user object is set.
Fixes bugs with attaching featured images to posts

This commit fixes two bugs in `WP_JSON_Media::attachThubmnail()`.

    - `attachThumbnail()` should dp nothing if `$update` is false since
    the post ID is needed.
    - The post ID must be fetched from the `$post` array.
Add RSD and Link headers for discovery

Fixes #40
Added unit testing skeleton with mostly stubbed server tests.
Link collection filtering docs to URL formatting guide

As noted in the compatibility document, URLs do not provide any standard
convention for passing query parameters as an array. Linking the collection
filtering guide to the article explaining the URL array syntax used in this
API will reduce confusion for new users.
Merge pull request #40 from WP-API/discovery

API discovery ala RSD
Introduce Response/ResponseInterface
Merge pull request #33 from WP-API/response-interface

Add response object
Merge pull request #54 from rachelbaker/issue-51

Added conditional to verify the $user object is set in prepare_author method.

Fixes #51
Merge pull request #49 from rachelbaker/multisite

Updated plugin activation and deactivation functions to be multisite compatible.

See #48
Merge pull request #65 from tierra/ticket-3-unit-tests

Added unit testing skeleton with mostly stubbed server tests.
Test all versions of PHP, with WP trunk
Add documentation on testing/developing
Don't check for IXR_Error
Remove IXR includes
Merge pull request #74 from kadamwhite/cross-link-docs

Link collection filtering docs to URL formatting guide
Remove hardcoded /pages references
Merge pull request #78 from WP-API/replace-hardcoded-pages-references

Remove hardcoded /pages references. Fixes #28.
Workaround for createFromFormat on PHP > 5.2