Linked images are uploaded on push


### Pull Request Summary

 1. persistent connections with Zendesk using `persistent_httparty`
 2. automatic uploads of images linked in markup
 3. automatic clean up of images previously uploaded
 4. using a local .ruby_version file

### Image Uploads

This feature works with Markdown and HTML. Images in the markup point to image files using file system paths. ZenPush handles the rest.

##### On Topic Creation...

 * The file system is searched for all relative image links in the HTML
 * A Zendesk "upload" is created for all images in the HTML
 * The Zendesk upload token is stored in an invisible `span` element in the topic HTML (does not modify local files)
 * Topic HTML is rewritten to point to Zendesk URLs for uploaded images (does not modify local files)

##### On  Topic Update...

 * On topic update, the upload token is retrieved from the existing topic HTML
 * The upload is deleted, thereby deleting all images previously uploaded
 * The upload process for Topic creation is invoked