user expects to attach/update screenshot to a requirement using InkFile

Given I'm on `new_proof_path`
When I upload a screenshot and
Then I expect to see a live preview and the screenshot on `requirements#index`

Attach proof
![screenshot 2014-03-26 17 50 36](https://cloud.githubusercontent.com/assets/331004/2531343/ae516ee4-b530-11e3-9aee-3e0fd30b8a9d.png)

Update proof
![screenshot 2014-03-26 17 49 44](https://cloud.githubusercontent.com/assets/331004/2531333/8d2c010c-b530-11e3-870b-5e5e35d0184d.png)

* using filepicker in proof forms
* extract proof form partial for new/edit views
* adding page header for new/edit views
* don't break Demeter's Law
* partial fix for specs, pending specs for new/edit
* display current proof image in edit page
* title for current image, moving out form container
* apply twitter bootstrap to proof form buttons
* preview attached image in new proof form
* extracting current proof image in a partial
* fix current image proof for edit proof form
* restrict services for filepicker to computer only
* adding httparty
* removing old image file when updating proof