Added a new app generator: praxis new app_name

This creates a blank directory structure ready to get going. It has everything from rackup and initialization, to basic directories, to spec configuration, etc…

Hello world example now is available through: praxis example app_name

At the same time, Thor classes and invocations have been totally reworked so that the Praxis cli can support a clean syntax, while each command can live in a different class, and such that the direct thor commands can still be executed outside the binary, but with similar namespaces.

Also, fixes #67 

Signed-off-by: Josep M. Blanquer <blanquer@rightscale.com>