Add method to determine what the latest version of an app is.

https://github.com/madewithbytes/us_ignite/issues/100

This is determined by the creation date of the version.
Add method to generate a unique signature for an version/app.

https://github.com/madewithbytes/us_ignite/issues/100

The signature is an MD5 of the content of the version or
application.
Validate app content before versioning.

https://github.com/madewithbytes/us_ignite/issues/100

Before versioning the application makes sure that the latest
version is different from the current application.