Add specs for PostStatusService

This implements all pending specs, and adds additional coverage for the
following functionality:

* Normal status creation
* Creating a reply status
* Creating a sensitive status
* Creating a status with spoiler text
* A status with no spoiler text gets an empty string for spoiler text
* Creating a status with custom visibility
* Creating a status for an application
* Processing mentions
* Processing Hashtags
* Pinging PuSH hubs
* Crawling links
* Attaching media