Checks that SECRET_KEY is set

At startup, checks that the `SECRET_KEY` configuration key is defined.
Add backbone.js and underscore.js
Add drag'n drop file support

User can now drag a file from another window (like a file explorer) and
drop it on a specific div in flaskup. UI updates are handled by a
backbone app.
Move javascript codebase to a Backbone app

Almost all javascript code has been moved inside the Backbone app. All
processing and UI updates are done via the Backbone app.
Move the readable_size function in the external js file
Post URL is defined at Backbone app creation

The URL where to POST the file is no more hard-codded in the Backbone
app. The URL must be defined when the app is instanciated.
Reorganize import of js files
Add jed.js (i18n javascript lib)
i18n for javascript strings
Small fix in error-message template
Compile .po files when building package
Update .gitignore files
Merge branch 'master' into jsmvc
Use Flask-Assets to manage css and js files
Small fixes in translations management
Merge branch 'master' of github.com:lmeunier/flaskup