Check get_post_type_object() returns an object before using it

This PR checks the return of get_post_type_object() in a few places where the code assumes it will return a post type object. It will [return NULL](http://codex.wordpress.org/Function_Reference/get_post_type_object#Return_Values) if a post type for a requested post is no longer registered (for instance, if a plugin has been disabled).

![wp-api-php-notice](https://cloud.githubusercontent.com/assets/2306629/5218322/5323abfc-7643-11e4-95ce-96315ac01cb9.png)

I scanned the code for get_post_type_object() and fixed all those instances where no checks were in place, with two exceptions.

1. [/lib/class-wp-json-posts.php:652](https://github.com/WP-API/WP-API/blob/develop/lib/class-wp-json-posts.php#L652) - The next line calls WP_JSON_Posts::check_read_permissions(). In the event of an unregistered post type, it will return before the $post_type var is used anway. However, it seems like the check_read_permissions() call could be made at the top of that method, before the call to get_post_type_object().

2. [/lib/class-wp-json-posts.php:915](https://github.com/WP-API/WP-API/blob/develop/lib/class-wp-json-posts.php#L915) - get_post_type_object() is called on a post type that is pulled from the database while updating a post. I left this one without a check because I am not familiar enough with the API yet to know whether the API would have returned an error before this step anyway. Will it block an update to a post type that is not registered?

The use-case which led me to this issue involved securing non-public posts that still need to be exposed to the API (provided other authentication checks are passed). If a post can be updated via the API for a type that is no longer registered, how would WordPress know which user permissions to apply to that action?